Multi-beam CSI reporting

ABSTRACT

Systems and methods for multi-beam Channel State Information (CSI) reporting are provided. In some embodiments, a method of operation of a second node connected to a first node in a wireless communication network for reporting multi-beam CSI includes reporting a rank indicator and a beam count indicator in a first transmission to the first node. The method also includes reporting a cophasing indicator in a second transmission to the first node. The cophasing indicator identifies a selected entry of a codebook of cophasing coefficients where the number of bits in the cophasing indicator is identified by at least one of the beam count indicator and the rank indicator. In this way, feedback for both a rank indicator and a beam count indicator may be possible which may allow robust feedback and variably sized cophasing and beam index indicators.

RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 national phase filing ofInternational Application No. PCT/IB2017/058320, filed Dec. 21, 2017,which claims the benefit of provisional patent application Ser. No.62/455,440, filed Feb. 6, 2017, the disclosures of which are herebyincorporated herein by reference in their entireties.

TECHNICAL FIELD

The present disclosure relates to multi-beam Channel State Information(CSI) reporting.

BACKGROUND

Because Physical Uplink Control Channel (PUCCH) payloads areconstrained, Long Term Evolution (LTE) defines Channel State Information(CSI) reporting types that carry subsets of CSI components (such asChannel Quality Indicators (Cal), Precoding Matrix Indicators (PMI),Rank Indicators (RI), and CSI-RS Resource Indicator (CRI)). Togetherwith the PUCCH reporting mode and ‘Mode State’, each reporting typedefines a payload that can be carried in a given PUCCH transmission,which is given in Third Generation Partnership Project (3GPP) TechnicalSpecification (TS) 36.213, Table 7.2.2-3. In Rel-13, all PUCCH reportingtypes have payloads that are less than or equal to 11 bits, and so allcan be carried in a single PUCCH Format 2 transmission.

SUMMARY

Systems and methods for multi-beam Channel State Information (CSI)reporting are provided. In some embodiments, a method of operation of asecond node connected to a first node in a wireless communicationnetwork for reporting multi-beam CSI includes reporting a rank indicatorand a beam count indicator in a first transmission to the first node.The method also includes reporting a cophasing indicator in a secondtransmission to the first node. The cophasing indicator identifies aselected entry of a codebook of cophasing coefficients where the numberof bits in the cophasing indicator is identified by at least one of thebeam count indicator and the rank indicator. In this way, feedback forboth a rank indicator and a beam count indicator may be possible whichmay allow robust feedback and variably sized cophasing and beam indexindicators.

In some embodiments, reporting the rank indicator and the beam countindicator in the first transmission includes reporting the rankindicator and the beam count indicator in a first transmission on anuplink control channel. Reporting the cophasing indicator in the secondtransmission includes reporting the cophasing indicator in the secondtransmission on the uplink control channel.

In some embodiments, the beam count indicator includes a number of beamsand/or an indication of relative powers between the beams, wherein abeam with zero power implicitly indicates an absence of the beam. Insome embodiments, the possible values of at least one of the beam countindicator and the cophasing indicator comprise both a zero and anon-zero value.

In some embodiments, the method also includes reporting a beam index ina third transmission to the first node. In some embodiments, the thirdtransmission also includes at least one of a beam rotation and/or asecond beam index.

In some embodiments, the method also includes jointly identifying anumber of beams and an index of a beam in a multi-beam CSI report. Thefirst transmission and the second transmission include transmitting themulti-beam CSI report to the first node. In some embodiments, jointlyidentifying the number of beams and indices of the beams in themulti-beam CSI report includes determining a number of beams L used toconstruct the multi-beam CSI report; and determining a beam indicatorfor an lth beam, the beam indicator identifying the index of a beam ofthe multi-beam CSI report if L is at least l, and otherwise identifyingthat L is less than l.

In some embodiments, the method also includes reporting CSIcorresponding to a first number of beams if the CSI corresponds to afirst rank; and reporting CSI corresponding to a second number of beamsif the CSI corresponds to a second rank. In some embodiments, the firstrank is smaller than the second rank and the first number of beams islarger than the second number of beams.

In some embodiments, the method also includes providing an indication ofat least one beam index pair index (l_(k),m_(k)) corresponding to a beamd(k).

In some embodiments, beam d(k) comprises a set of complex numbers, eachelement of the set of complex numbers being characterized by at leastone complex phase shift such that:

d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))),and d_(i)(k) are the n^(th) and l^(th) elements of the beam d(k),respectively, α_(i,n) is a real number corresponding to the l^(th) andn^(th) elements of the beam d(k), p and q are integers, and Δ_(1,k) andΔ_(2,k) are real numbers corresponding to beam directions of the twodimensional beam d(k) that determine the complex phase shifts e^(j2πΔ)^(1,k) and e^(j2πΔ) ^(2,k) in the first and the second dimension,respectively.

In some embodiments, a method of operation of a first node connected toa second node in a wireless communication network for receivingmulti-beam CSI includes receiving a rank indicator and a beam countindicator in a first transmission from the second node; and receiving acophasing indicator in a second transmission from the second node. Thecophasing indicator identifies a selected entry of a codebook ofcophasing coefficients wherein the number of bits in the cophasingindicator is identified by at least one of the beam count indicator andthe rank indicator.

In some embodiments, receiving the rank indicator and the beam countindicator in the first transmission includes receiving the rankindicator and the beam count indicator in a first transmission on anuplink control channel; and receiving the cophasing indicator in thesecond transmission includes receiving the cophasing indicator in thesecond transmission on the uplink control channel.

In some embodiments, the beam count indicator includes at least one of anumber of beams and/or an indication of relative powers.

In some embodiments, the possible values of at least one of the beamcount indicator, and the cophasing indicator include both a zero and anon-zero value.

In some embodiments, the method also includes receiving a beam index ina third transmission from the second node. In some embodiments, thethird transmission also includes at least one of the group consisting ofa beam rotation and second beam index.

In some embodiments, the method also includes receiving CSIcorresponding to a first number of beams if the CSI corresponds to afirst rank; and receiving CSI corresponding to a second number of beamsif the CSI corresponds to a second rank. In some embodiments, the firstrank is smaller than the second rank and the first number of beams islarger than the second number of beams.

In some embodiments, the method also includes receiving an indication ofat least one beam index pair index (l_(k),m_(k)) corresponding to a beamd(k).

In some embodiments, each beam d(k) comprises a set of complex numbersand each element of the set of complex numbers being characterized by atleast one complex phase shift such that

d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))),d_(n)(k), and d_(i)(k) are the n^(th) and i^(th) elements of the beamd(k), respectively, α_(i,n) is a real number corresponding to the i^(th)and n^(th) elements of the beam d(k), p and q are integers, and Δ_(1,k)and Δ_(2,k) are real numbers corresponding to the beam directions of thetwo dimensional beam d(k) that determine the complex phase shiftse^(j2πΔ) ^(1,k) and e^(j2πΔ) ^(2,k) respectively.

In some embodiments, a second node includes at least one processor andmemory. The memory includes instructions executable by the at least oneprocessor whereby the second node is operable to report a rank indicatorand a beam count indicator in a first transmission to a first node; andreport a cophasing indicator in a second transmission to the first node.The cophasing indicator identifying a selected entry of a codebook ofcophasing coefficients wherein the number of bits in the cophasingindicator is identified by at least one of the beam count indicator andthe rank indicator.

In some embodiments, a second node includes a reporting module operableto report a rank indicator and a beam count indicator in a firsttransmission to a first node; and report a cophasing indicator in asecond transmission to the first node. The cophasing indicatoridentifying a selected entry of a codebook of cophasing coefficientswherein the number of bits in the cophasing indicator is identified byat least one of the beam count indicator and the rank indicator.

In some embodiments, a first node includes at least one processor andmemory. The memory includes instructions executable by the at least oneprocessor whereby the first node is operable to: receive a rankindicator and a beam count indicator in a first transmission from asecond node; and receive a cophasing indicator in a second transmissionfrom the second node. The cophasing indicator identifying a selectedentry of a codebook of cophasing coefficients wherein the number of bitsin the cophasing indicator is identified by at least one of the beamcount indicator and the rank indicator.

In some embodiments, a first node includes a receiving module operableto receive a rank indicator and a beam count indicator in a firsttransmission from a second node; and receive a cophasing indicator in asecond transmission from the second node. The cophasing indicatoridentifying a selected entry of a codebook of cophasing coefficientswherein the number of bits in the cophasing indicator is identified byat least one of the beam count indicator and the rank indicator.

In some embodiments, the first node is a radio access node. In someembodiments, the second node is a wireless device. In some embodiments,the wireless communication network is a Long Term Evolution (LTE)wireless communication network. In some embodiments, the wirelesscommunication network is a New Radio (NR) or Fifth Generation (5G)wireless communication network.

In some embodiments, in Third Generation Partnership Project (3GPP), foradvanced CSI reporting in Rel-14, W₁, which contains information of thebeam indices, is reported with a payload of 13 bits while W₂, whichcontains information of the cophasing coefficients, is reported with apayload of 6 bits for rank=1 or 12 bits for rank=2. This implicitlyassumes aperiodic reporting on a Physical Uplink Shared Channel (PUSCH)where the feedback payload is not constrained. For periodic CSIreporting on a Physical Uplink Control Channel (PUCCH), though, LongTerm Evolution (LTE) currently only supports CSI feedback on PUCCHFormat 2, which has a payload of 11 bits. Neither W₁ nor W₂ (in the caseof rank-2) can be directly reported on a single PUCCH Format 2transmission since the payload is larger than 11 bits.

Indications of W₁ and W₂ for the advanced CSI codebook in 3GPP are (atleast in some cases) larger than can be supported on PUCCH format 2, soadvanced CSI is not yet supported adequately for PUCCH reporting.

Some embodiments disclosed herein relate to:

Subsampling W₂ by linking two cophasing vectors (one for each layer) inrank 2 such that the two vectors are orthogonal and using the QuadraturePhase-Shift Keying (QPSK) alphabet for each cophasing coefficient, whichresults in 4 bits for W₂ feedback.

Subsampling W₂ by using the same cophasing coefficients for twopolarizations with independent cophasing vectors in rank 2 and using theBinary Phase-Shift Keying (BPSK) alphabet for each cophasingcoefficient, which results in 4 bits for W₂ feedback.

Feeding back both a rank indicator and a beam count indicator in a PUCCHtransmission to allow robust feedback, and to allow a variably sizedcophasing and beam index indicators to be carried on PUCCH.

Some embodiments relate to constructing a feedback mechanism forreporting rich CSI feedback on small payload channels, such as PUCCH,while still maintaining sufficient CSI accuracy and reliability. In someembodiments, this is accomplished through various mechanisms, includingthose that report on subsets of codebooks, use variably sized indicatorsfor CSI reporting components, and multiplexing compatible CSI componentstogether. These embodiments allow periodic feedback of advanced CSI onexisting PUCCH format 2.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing figures incorporated in and forming a part ofthis specification illustrate several aspects of the disclosure, andtogether with the description serve to explain the principles of thedisclosure.

FIG. 1 illustrates a wireless communication system according to someembodiments;

FIG. 2 illustrates a downlink physical resource such as may be used in aLong Term Evolution (LTE) wireless communication system;

FIG. 3 illustrates a time-domain structure as may be used in the LTEwireless communication system;

FIG. 4 illustrates a Downlink subframe as may be used in the LTEwireless communication system;

FIG. 5 illustrates uplink L1/L2 control signaling transmission on aPhysical Uplink Control Channel (PUCCH), according to some embodimentsof the present disclosure;

FIG. 6 illustrates a transmission structure of a precoded spatialmultiplexing mode as may be used in the LTE wireless communicationsystem according to some embodiments of the present disclosure;

FIG. 7 illustrates an example comparison of a subband and a widebandaccording to some embodiments of the present disclosure;

FIG. 8 illustrates an example two-dimensional antenna array according tosome embodiments of the present disclosure;

FIG. 9A illustrates an example of oversampled Discrete Fourier Transform(DFT) beams with (N₁, N₂)=(4,2) and (O₁, O₂)=(4,4) according to someembodiments of the present disclosure. FIG. 9B illustrates an examplefor W1 beam selection, W1 beam power, and W2 determination according tosome embodiments of the present disclosure;

FIGS. 10A, 11A, 12A, and 13A illustrate procedures for reporting CSIfeedback on a physical channel according to some embodiments of thepresent disclosure;

FIGS. 10B, 11B, 12B, and 13B illustrate procedures for receiving CSIfeedback on a physical channel according to some embodiments of thepresent disclosure;

FIGS. 14 and 15 illustrate example embodiments of a wireless deviceaccording to some embodiments of the present disclosure; and

FIGS. 16 through 18 illustrate example embodiments of a radio networknode according to some embodiments of the present disclosure.

DETAILED DESCRIPTION

The embodiments set forth below represent information to enable thoseskilled in the art to practice the embodiments and illustrate the bestmode of practicing the embodiments. Upon reading the followingdescription in light of the accompanying drawing figures, those skilledin the art will understand the concepts of the disclosure and willrecognize applications of these concepts not particularly addressedherein. It should be understood that these concepts and applicationsfall within the scope of the disclosure.

Note that although terminology from 3GPP LTE has been used in thisdisclosure, this should not be seen as limiting the scope of thedisclosure to only the aforementioned system. Other wireless systems,including New Radio (NR) (i.e., Fifth Generation (5G)), WidebandCode-Division Multiple Access (WCDMA), Worldwide Interoperability forMicrowave Access (WiMax), Ultra Mobile Broadband (UMB), and GlobalSystem for Mobile Communications (GSM), may also benefit from exploitingthe ideas covered within this disclosure.

Also note that terminology such as evolved or enhanced NodeB (eNodeB)and User Equipment (UE) should be considered non-limiting and does notimply a certain hierarchical relation between the two; in general“eNodeB” could be considered as device 1 and “UE” device 2, and thesetwo devices communicate with each other over some radio channel. Herein,wireless transmissions in the downlink are discussed in detail, but someembodiments of the disclosure are equally applicable in the uplink.

In this regard, FIG. 1 illustrates one example of a wireless system 10(e.g., a cellular communications system) in which embodiments of thepresent disclosure may be implemented. The wireless system 10 includes afirst node 12, which in this example is a radio access node. However,the first node 12 is not limited to a radio access node and can beanother device such as a general radio node allowing communicationwithin a radio network, including a wireless device as described below.The radio access node 12 provides wireless access to other nodes such aswireless devices or other access nodes, such as a second node 14, withina coverage area 16 (e.g., cell) of the radio access node 12. In someembodiments, the second node 14 is a Long Term Evolution User Equipment(LTE UE). Note that the term “UE” is used herein in its broad sense tomean any wireless device. As such, the terms “wireless device” and “UE”are used interchangeably herein.

LTE uses Orthogonal Frequency-Division Multiplexing (OFDM) in thedownlink and Discrete Fourier Transform (DFT)-spread OFDM in the uplink.The basic LTE downlink physical resource can thus be seen as atime-frequency grid as illustrated in FIG. 2, where each resourceelement corresponds to one OFDM subcarrier during one OFDM symbolinterval.

FIG. 3 illustrates a time-domain structure as may be used in the LTEwireless communication system. In the time domain, LTE downlinktransmissions are organized into radio frames of 10 ms, each radio frameconsisting of ten equally-sized subframes of length T_(subframe)=1 ms.

Furthermore, the resource allocation in LTE is typically described interms of resource blocks, where a resource block corresponds to one slot(0.5 ms) in the time domain and twelve contiguous subcarriers in thefrequency domain. Resource blocks are numbered in the frequency domain,starting with 0 from one end of the system bandwidth.

Downlink transmissions are dynamically scheduled; i.e., in each subframethe base station transmits control information regarding to whichterminals data is transmitted and upon which resource blocks the data istransmitted in the current downlink subframe. This control signaling istypically transmitted in the first 1, 2, 3 or 4 OFDM symbols in eachsubframe. A downlink system with 3 OFDM symbols as control isillustrated in FIG. 4.

LTE uses Hybrid Automatic Repeat Requests (HARQ), where after receivingdownlink data in a subframe, the terminal attempts to decode it andreports to the base station whether the decoding was successful (ACK) ornot (NACK). In case of an unsuccessful decoding attempt, the basestation can retransmit the erroneous data.

Uplink control signaling from the terminal to the base station consistsof:

-   -   HARQ acknowledgements for received downlink data;    -   terminal reports related to the downlink channel conditions,        used as assistance for the downlink scheduling;    -   scheduling requests, indicating that a mobile terminal needs        uplink resources for uplink data transmissions.

In order to provide frequency diversity, these frequency resources arefrequency hopping on the slot boundary, i.e., one “resource” consists of12 subcarriers at the upper part of the spectrum within the first slotof a subframe and an equally sized resource at the lower part of thespectrum during the second slot of the subframe or vice versa. If moreresources are needed for the uplink L1/L2 control signaling, e.g., incase of very large overall transmission bandwidth supporting a largenumber of users, additional resource blocks can be assigned next to thepreviously assigned resource blocks. FIG. 5 illustrates uplink L1/L2control signaling transmission on a Physical Uplink Control Channel(PUCCH),

As mentioned above, uplink L1/L2 control signaling includes HARQacknowledgements, channel state information and scheduling requests.Different combinations of these types of messages are possible asdescribed further below, but to explain the structure for these cases itis beneficial to discuss separate transmission of each of the typesfirst, starting with the HARQ and the scheduling request. There are fiveformats defined for the PUCCH in Rel-13, each capable of carrying adifferent number of bits. For this background art, PUCCH formats 2 and 3are the most relevant.

UEs can report channel state information (CSI) to provide the eNodeBwith an estimate of the channel properties at the terminal in order toaid channel-dependent scheduling. Such channel properties are those thattend to vary with the fading of the channel or with interference, suchas the relative gain and phase of the channel between antenna elements,the signal to interference and noise ratio (SINR) in a given subframe,etc. Such CSI feedback is used to adapt Multiple-Input Multiple-Output(MIMO) precoding and modulation and coding states. LTE provides othermeasures of channel properties, such as Received Signal StrengthIndicators (RSSI), Reference Signal Received Power (RSRP), and ReferenceSignal Received Quality (RSRQ); however, these are longer termproperties not used to adapt MIMO transmission or to select modulationand coding states, and so are not considered CSI in the context of thisdisclosure.

A CSI report consists of multiple bits per subframe transmitted in theuplink control information (UCI) report. PUCCH Format 1, which iscapable of at most two bits of information per subframe, can obviouslynot be used for this purpose. Transmission of CSI reports on the PUCCHin Rel-13 is instead handled by PUCCH Formats 2, 3, 4, and 5, which arecapable of multiple information bits per subframe.

PUCCH Format 2 resources are semi-statically configured. A Format 2report can carry a payload of at most 11 bits. Variants of Format 2 areFormat 2a and 2b which also carry HARQ-ACK information of 1 and 2 bits,respectively for a normal cyclic prefix. For an extended cyclic prefix,PUCCH Format 2 can also carry HARQ-ACK information. For simplicity, theyare all referred to as Format 2 herein.

PUCCH format 3 is designed to support larger HARQ-ACK payloads and cancarry up to 10 or 20 HARQ-ACK bits for FDD and TDD, respectively. It canalso carry Scheduling Requests (SR), and therefore supports up to 21bits total. PUCCH format 3 can also carry CSI. PUCCH formats 4 and 5carry still larger payloads.

Because PUCCH payloads are constrained, LTE defines CSI reporting typesthat carry subsets of CSI components (such as Channel Quality Indicators(CQI), Precoding Matrix Indicators (PMI), Rank Indicators (RI), andCSI-RS Resource Indicators (CRI)). Together with the PUCCH reportingmode and ‘Mode State,’ each reporting type defines a payload that can becarried in a given PUCCH transmission, which is given in 3GPP TS 36.213,Table 7.2.2-3. In Rel-13, all PUCCH reporting types have payloads thatare less than or equal to 11 bits, therefore all can be carried in asingle PUCCH Format 2 transmission.

Various CSI reporting types are defined in Rel-13 LTE:

-   -   Type 1 report supports CQI feedback for the UE selected subbands    -   Type 1a report supports subband CQI and second PMI feedback    -   Type 2, Type 2b, and Type 2c reports support wideband CQI and        PMI feedback    -   Type 2a report supports wideband PMI feedback    -   Type 3 report supports RI feedback    -   Type 4 report supports wideband CQI    -   Type 5 report supports RI and wideband PMI feedback    -   Type 6 report supports RI and PMI feedback    -   Type 7 report support CRI and RI feedback    -   Type 8 report supports CRI, RI and wideband PMI feedback    -   Type 9 report supports CRI, RI and PMI feedback    -   Type 10 report supports CRI feedback

These reporting types are transmitted on PUCCH with periodicities andoffsets (in units of subframes) determined according to whether CQI,Class A first PMI, RI, or CRI are carried by the reporting type.

Table 1 below shows the subframes when the various reporting types aretransmitted assuming that wideband CSI reports are used with a singleCSI subframe set. Similar mechanisms are used for subband reporting andfor multiple subframe sets.

TABLE 1 PUCCH Report Transmission Time for CSI Reporting Types CSI CSISubframe in which wideband CSI content Reporting Type reporting type(s)are transmitted CQI 1, 1a, 2, (10 × n_(f) + [n_(s)/2] − 2b, 2c, 4N_(OFFSET, CQI))mod (N_(pd)) = 0 Class A first PMI 2a (10 × n_(f) +[n_(s)/2] − N_(OFFSET, CQI))mod (H′ · N_(pd)) = 0 RI 3, 5 (10 × n_(f) +[n_(s)/2] − N_(OFFSET, CQI) − N_(OFFSET, RI))mod (N_(pd) · M_(RI)) = 0CRI* 7, 8, 9, 10 (10 × n_(f) + [n_(s)/2] − N_(OFFSET, CQI) −N_(OFFSET, RI))mod (N_(pd) · M_(RI) · M_(CRI)) = 0

Note that CRI is for the case where more than one CSI-RS resource isconfigured. Where (as defined in 3GPP TSs 36.213 and 36.331):

-   -   n_(f) is the system frame number    -   n_(s) is the slot number within a radio frame    -   N_(pd) is a periodicity in subframes set by the higher layer        parameter cqi-pmi-ConfigIndex    -   N_(OFFSET,CQI) is an offset in subframes set by the higher layer        parameter cqi-pmi-ConfigIndex    -   H′ is set by the higher layer parameter periodicityFactorWB    -   M_(RI) is periodicity multiple in subframes set by the higher        layer parameter ri-ConfigIndex    -   N_(OFFSET,RI) is an offset in subframes set by the higher layer        parameter ri-ConfigIndex    -   M_(CRI) is periodicity multiple in subframes set by the higher        layer parameter cri-ConfigIndex

PUCCH CSI reporting has a fundamental periodicity of N_(pd) subframes,and CQIs can be reported at this rate. If an RI is configured, it canalso be reported at the same rate as CQI by configuring M_(RI)=1, sincean offset N_(OFFSET,RI) can allow the RI to have different subframeshifts of the same periodicity as the CQI. On the other hand, a Class Afirst PMI is time multiplexed with the CQI, in which the Class A firstPMI is transmitted instead of the CQI in one out of H′ transmissions ofthe CQI. The CRI is time multiplexed with the RI in a similar way, i.e.,the CRI is transmitted instead of the RI in one out of M_(CRI)transmissions of the RI.

Also, PUCCH Format 3 can carry ACK/NACK and CSI in the same PUCCHtransmission, but the CSI must be from only one serving cell.Furthermore, in Rel-13, a UE only transmits CSI on PUCCH Format 3 whentransmitting ACK/NACK. If there is no ACK/NACK to be transmitted in agiven subframe and CSI is to be transmitted on PUCCH, the UE will usePUCCH Format 2 in that subframe.

LTE control signaling can be carried in a variety of ways, includingcarrying control information on a Physical Downlink Control Channel(PDCCH), Enhanced Physical Downlink Control Channel (EPDCCH) or PUCCH,embedded in a (PUSCH), in Medium Access Control (MAC) control elements(‘MAC CEs’), or in Radio Resource Control (RRC) signaling. Each of thesemechanisms is customized to carry a particular kind of controlinformation. As used herein, a control channel may refer to any of thesemechanisms. Additionally, a transmission on a control channel may referto a separate transmission that carries the information or a part of atransmission that carries specific information.

Control information carried on the PDCCH, EPDCCH, PUCCH, or embedded inPUSCH is physical layer related control information, such as DownlinkControl Information (DCI), Uplink Control Information (UCI), asdescribed in 3GPP TS 36.211, 36.212, and 36.213. DCI is generally usedto instruct the UE to perform some physical layer function, providingthe needed information to perform the function. UCI generally providesthe network with needed information, such as HARQ-ACK, SchedulingRequest (SR), Channel State Information (CSI), including CQI, PMI, RI,and/or CRI. UCI and DCI can be transmitted on a subframe-by-subframebasis, and so are designed to support rapidly varying parameters,including those that can vary with a fast fading radio channel. BecauseUCI and DCI can be transmitted in every subframe, UCI or DCIcorresponding to a given cell tend to be on the order of tens of bits,in order to limit the amount of control overhead.

Control information carried in MAC CEs is carried in MAC headers on theUplink and Downlink Shared Transport Channels (UL-SCH and DL-SCH), asdescribed in 3GPP TS 36.321. Since a MAC header does not have a fixedsize, control information in MAC CEs can be sent when it is needed anddoes not necessarily represent a fixed overhead. Furthermore, MAC CEscan carry larger control payloads efficiently, since they are carried inUL-SCH or DL-SCH transport channels, which benefit from link adaptation,HARQ, and can be turbo coded (whereas UCI and DCI cannot be in Rel-13).MAC CEs are used to perform repetitive tasks that use a fixed set ofparameters, such as maintaining timing advance or buffer statusreporting, but these tasks generally do not require transmission of aMAC CE on a subframe-by-subframe basis. Consequently, channel stateinformation related to a fast fading radio channel, such as PMIs, CQIs,RIs, and CRIs are not carried in MAC CEs in Rel-13.

Dedicated RRC control information is also carried through UL-SCHs andDL-SCHs using Signaling Radio Bearers (SRBs), as discussed in 3GPP TS36.331. Consequently, it can also carry large control payloadsefficiently. However, SRBs are not generally intended for very frequenttransmission of large payloads, and need to be available to support lessfrequent signaling that should be highly reliably transmitted, such asfor mobility procedures including handover. Therefore, similar to theMAC, RRC signaling does not carry channel state information related to afast fading radio channel, such as PMIs, CQIs, RIs, and CRIs in Rel-13.In fact, this kind of CSI is only carried in UCI signaling on PUSCHs orPUCCHs.

Multi-antenna techniques can significantly increase the data rates andreliability of a wireless communication system. The performance is inparticular improved if both the transmitter and the receiver areequipped with multiple antennas, which results in a Multiple-InputMultiple-Output (MIMO) communication channel. Such systems and/orrelated techniques are commonly referred to as MIMO.

The LTE standard is currently evolving with enhanced MIMO support. Acore component in LTE is the support of MIMO antenna deployments andMIMO related techniques. LTE Release 12 supports an 8-layer spatialmultiplexing mode for 8 Tx antennas with channel dependent precoding.The spatial multiplexing mode is aimed for high data rates in favorablechannel conditions. An illustration of the spatial multiplexingoperation is provided in FIG. 6.

As seen in FIG. 6, the information carrying symbol vector s ismultiplied by an N_(T)×r precoder matrix W, which serves to distributethe transmit energy in a subspace of the N_(T) (corresponding to N_(T)antenna ports) dimensional vector space. The precoder matrix istypically selected from a codebook of possible precoder matrices, and istypically indicated by means of a PMI, which specifies a unique precodermatrix in the codebook for a given number of symbol streams. The rsymbols in s each correspond to a layer, and r is referred to as thetransmission rank. In this way, spatial multiplexing is achieved, sincemultiple symbols can be transmitted simultaneously over the sameTime/Frequency Resource Element (TFRE). The number of symbols r istypically adapted to suit the current channel properties.

LTE uses OFDM in the downlink (and DFT precoded OFDM in the uplink), andhence the received N_(R)×1 vector y_(n) for a certain TFRE on subcarriern (or alternatively data TFRE number n) is thus modeled by:y _(n) =H _(n) Ws _(n) +e _(n)  Equation 1where e_(n) is a noise/interference vector obtained as realizations of arandom process. The precoder W can be a wideband precoder, which isconstant over frequency, or frequency selective.

The precoder matrix W is often chosen to match the characteristics ofthe N_(R)×N_(T) MIMO channel matrix H_(n), resulting in so-calledchannel dependent precoding. This is also commonly referred to asclosed-loop precoding and essentially strives for focusing the transmitenergy into a subspace which is strong in the sense of conveying much ofthe transmitted energy to the UE. In addition, the precoder matrix mayalso be selected to strive for orthogonalizing the channel, meaning thatafter proper linear equalization at the UE, the inter-layer interferenceis reduced.

One example method for a UE to select a precoder matrix W can be toselect the W_(k) that maximizes the Frobenius norm of the hypothesizedequivalent channel:

$\begin{matrix}{\max\limits_{k}{{{\hat{H}}_{n}W_{k}}}_{F}^{2}} & {{Equation}\mspace{14mu} 2}\end{matrix}$where Ĥ_(n) is a channel estimate, possibly derived from CSI-RS asdescribed below. W_(k) is a hypothesized precoder matrix with index k.Ĥ_(n)W_(k) is the hypothesized equivalent channel.

With regard to CSI feedback, a subband is defined as a number ofadjacent Physical Resource Block (PRB) pairs. In LTE, the subband size(i.e., the number of adjacent PRB pairs) depends on the systembandwidth, whether CSI reporting is configured to be periodic oraperiodic, and feedback type (i.e., whether higher layer configuredfeedback or UE-selected subband feedback is configured). An exampleillustrating the difference between subband and wideband is shown inFIG. 7. In the example, the subband consists of 6 adjacent PRBs. Notethat only two subbands are shown in FIG. 7 for simplicity ofillustration. Generally, all the PRB pairs in the system bandwidth aredivided into different subbands where each subband consists of a fixednumber of PRB pairs. In contract, wideband involves all the PRB pairs inthe system bandwidth. As mentioned above, a UE may feedback a singleprecoder that takes into account the measurements from all PRB pairs inthe system bandwidth if it is configured to report wideband PMI by theeNodeB. Alternatively, if the UE is configured to report subband PMI, aUE may feedback multiple precoders with one precoder per subband. Inaddition to the subband precoders, the UE may also feedback the widebandPMI.

In closed-loop precoding for the LTE downlink, the UE transmits, basedon channel measurements in the forward link (downlink), recommendationsto the eNodeB of a suitable precoder to use. The eNB configures the UEto provide feedback according to the UE's transmission mode, and maytransmit CSI-RS and configure the UE to use measurements of CSI-RS tofeedback recommended precoding matrices that the UE selects from acodebook. A single precoder that is supposed to cover a large bandwidth(wideband precoding) may be fed back. It may also be beneficial to matchthe frequency variations of the channel and instead feedback afrequency-selective precoding report, e.g. several precoders, one persubband. This is an example of the more general case of channel stateinformation feedback, which also encompasses feeding back otherinformation than recommended precoders to assist the eNodeB insubsequent transmissions to the UE. Such other information may includeCQIs as well as transmission RIs.

Given the CSI feedback from the UE, the eNodeB determines thetransmission parameters it wishes to use to transmit to the UE,including the precoding matrix, transmission rank, and Modulation andCoding State (MCS). These transmission parameters may differ from therecommendations the UE makes. Therefore, a rank indicator and MCS may besignaled in DCI, and the precoding matrix can be signaled in DCI or theeNodeB can transmit a demodulation reference signal from which theequivalent channel can be measured. The transmission rank, and thus thenumber of spatially multiplexed layers, is reflected in the number ofcolumns of the precoder W. For efficient performance, it is importantthat a transmission rank that matches the channel properties isselected.

In closed loop MIMO transmission schemes such as TM9 and TM10, a UEestimates and feeds the downlink CSI back to the eNodeB. The eNB usesthe feedback CSI to transmit downlink data to the UE. The CSI consistsof a transmission RI, a PMI and a CQI. A codebook of precoding matricesis used by the UE to find out the best match between the estimateddownlink channel H_(n) and a precoding matrix in the codebook based oncertain criteria, for example, the UE throughput. The channel H_(n) isestimated based on a Non-Zero Power CSI Reference Signal (NZP CSI-RS)transmitted in the downlink for TM9 and TM10.

The CQI/RI/PMI together provide the downlink channel state to the UE.This is also referred to as implicit CSI feedback since the estimationof H_(n) is not fed back directly. The CQI/RI/PMI can be wideband orsubband depending on which reporting mode is configured.

The RI corresponds to a recommended number of streams that are to bespatially multiplexed and thus transmitted in parallel over the downlinkchannel. The PMI identifies a recommended precoding matrix codeword (ina codebook which contains precoders with the same number of rows as thenumber of CSI-RS ports) for the transmission, which relates to thespatial characteristics of the channel. The CQI represents a recommendedtransport block size (i.e., code rate) and LTE supports transmission ofone or two simultaneous (on different layers) transmissions of transportblocks (i.e. separately encoded blocks of information) to a UE in asubframe. There is thus a relation between a CQI and an SINR of thespatial stream(s) over which the transport block or blocks aretransmitted.

Codebooks of up to 16 antenna ports have been defined in LTE Up toRelease 13. Both one dimension (1D) and two-dimension (2D) antennaarrays are supported. For LTE Release 12 UE and earlier, only a codebookfeedback for a 1D port layout is supported, with 2, 4, or 8 antennaports. Hence, the codebook is designed assuming these ports are arrangedon a straight line in one dimension. In LTE Rel-13, codebooks for 2Dport layouts were specified for the case of 8, 12, or 16 antenna ports.In addition, a codebook for 1D port layout for the case of 16 antennaports was also specified in LTE Rel-13.

In LTE Rel-13, two types of CSI reporting were introduced, i.e., Class Aand Class B. In Class A CSI reporting, a UE measures and reports CSIbased on a new codebook for the configured 2D antenna array with 8, 12or 16 antenna ports. The Class A codebook is defined by five parameters,i.e. (N1,N2,Q1,Q2,CodebookConfig), where (N1,N2) are the number ofantenna ports in a first and a second dimension, respectively. (Q1,Q2)are the DFT oversampling factor for the first and the second dimension,respectively. CodebookConfig ranges from 1 to 4 and defines fourdifferent ways the codebook is formed. For CodebookConfig=1, a PMIcorresponding to a single 2D beam is fed back for the whole systembandwidth while for CodebookConfig={2,3,4}, PMIs corresponding to four2D beams are fed back and each subband may be associated with adifferent 2D beam. The CSI consists of a RI, a PMI and a CQI or CQIs,similar to the CSI reporting in pre Rel-13.

In Class B CSI reporting, in one scenario (also referred to as“K_(CSI-RS)>1”), the eNB may pre-form multiple beams in one antennadimension. There can be multiple ports (1, 2, 4, or 8 ports) within eachbeam on the other antenna dimension. “Beamformed” CSI-RS are transmittedalong each beam. A UE first selects the best beam from a group of beamsconfigured and then measures CSI within the selected beam based on thelegacy pre-Release 13 LTE codebook for 2, 4, or 8 ports. The UE thenreports back the selected beam index and the CSI corresponding to theselected beam. In another scenario (also referred to as “K_(CSI-RS)=1”),the eNB may form up to 4 (2D) beams on each polarization and“beamformed” CSI-RS is transmitted along each beam. A UE measures CSI onthe “beamformed” CSI-RS and feedback CSI based on a new Class B codebookfor 2, 4, or 8 ports.

In LTE Release-10, a new reference symbol sequence was introduced forthe intent to estimate downlink channel state information, the CSI-RS.The CSI-RS provides several advantages over basing the CSI feedback onthe CRS which were used, for that purpose, in previous releases.Firstly, the CSI-RS is not used for demodulation of the data signal, andthus does not require the same density (i.e., the overhead of the CSI-RSis substantially less). Secondly, CSI-RS provides a much more flexiblemeans to configure CSI feedback measurements (e.g., which CSI-RSresource to measure on can be configured in a UE specific manner).

By measuring a CSI-RS transmitted from the eNodeB, a UE can estimate theeffective channel the CSI-RS is traversing including the radiopropagation channel and antenna gains. In more mathematical rigor thisimplies that if a known CSI-RS signal x is transmitted, a UE canestimate the coupling between the transmitted signal and the receivedsignal (i.e., the effective channel). Hence if no virtualization isperformed in the transmission, the received signal y can be expressedas:y=Hx+e  Equation 3and the UE can estimate the effective channel H.

Up to eight CSI-RS ports can be configured in LTE Rel-10, that is, theUE can estimate the channel from up to eight transmit antenna ports. InLTE Release 13, the number of CSI-RS ports that can be configured isextended to up to sixteen ports (3GPP TS 36.213, 3GPP TS 36.211). In LTERelease 14, supporting up to 32 CSI-RS ports is under consideration.

Related to CSI-RS is the concept of zero-power CSI-RS resources (alsoknown as a muted CSI-RS) that are configured just as regular CSI-RSresources, so that a UE knows that the data transmission is mappedaround those resources. The intent of the zero-power CSI-RS resources isto enable the network to mute the transmission on the correspondingresources in order to boost the SINR of a corresponding non-zero powerCSI-RS, possibly transmitted in a neighbor cell/transmission point. ForRel-11 of LTE a special zero-power CSI-RS was introduced that a UE ismandated to use for measuring interference plus noise. A UE can assumethat the Transmission Points (TPs) of interest are not transmitting onthe zero-power CSI-RS resource, and the received power can therefore beused as a measure of the interference plus noise.

Based on a specified CSI-RS resource and on an interference measurementconfiguration (e.g. a zero-power CSI-RS resource), the UE can estimatethe effective channel and noise plus interference, and consequently alsodetermine the rank, precoding matrix, and MCS to recommend to best matchthe particular channel.

Some embodiments of the current disclosure may be used with twodimensional antenna arrays, and some of the presented embodiments usesuch antennas. Such antenna arrays may be (partly) described by thenumber of antenna columns corresponding to the horizontal dimensionN_(h), the number of antenna rows corresponding to the verticaldimension N_(v) and the number of dimensions corresponding to differentpolarizations N_(p). The total number of antennas is thus N=N_(h) N_(v)N_(p). It should be pointed out that the concept of an antenna isnon-limiting in the sense that it can refer to any virtualization (e.g.,linear mapping) of the physical antenna elements. For example, pairs ofphysical sub-elements could be fed the same signal, and hence share thesame virtualized antenna port.

An example of a 4×4 array with cross-polarized antenna elements isillustrated in FIG. 8.

Precoding may be interpreted as multiplying the signal with differentbeamforming weights for each antenna prior to transmission. A typicalapproach is to tailor the precoder to the antenna form factor, i.e.taking into account N_(h), N_(v) and N_(p) when designing the precodercodebook. Such 2D codebooks may not strictly relate vertical orhorizontal dimensions to the dimensions that antenna ports areassociated with. Therefore, 2D codebooks can be considered to have afirst and a second number of antenna ports N₁ and N₂, wherein N₁ cancorrespond to either the horizontal or vertical dimension, and so N₂corresponds to the remaining dimension. That is, if N₁=N_(h), thenN₂=N_(v), while if N₁=N_(v), then N₂=N_(h). Similarly, 2D codebooks maynot strictly relate antenna ports to polarization, and be designed withcophasing mechanisms used to combine two beams or two antenna ports, asdescribed in the following.

A common type of precoding is to use a DFT-precoder, where the precodervector used to precode a single-layer transmission using asingle-polarized uniform linear array (ULA) with N₁ antennas is definedas:

$\begin{matrix}{{w_{1\; D}\left( {l,N_{1},O_{1}} \right)} = {\frac{1}{\sqrt{N_{1}}}\begin{bmatrix}e^{j\; 2{\pi \cdot 0 \cdot \frac{l}{O_{1}N_{1}}}} \\e^{j\; 2{\pi \cdot 1 \cdot \frac{l}{O_{1}N_{1}}}} \\\vdots \\e^{j\; 2{\pi \cdot {({N_{1} - 1})} \cdot \frac{l}{O_{1}N_{1}}}}\end{bmatrix}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$where l=0, 1, . . . , O₁N₁−1 is the precoder index and O₁ is an integeroversampling factor. A precoder for a dual-polarized Uniform LinearArray (ULA) with N₁ antennas per polarization (and so 2N₁ antennas intotal) can be similarly defined as:

$\begin{matrix}{{w_{{1\; D},{DP}}\left( {l,N_{1},O_{1}} \right)} = {\begin{bmatrix}{w_{1D}(l)} \\{e^{j\;\phi}{w_{1D}(l)}}\end{bmatrix} = {\begin{bmatrix}{w_{1D}(l)} & 0 \\0 & {w_{1D}(l)}\end{bmatrix}\begin{bmatrix}1 \\e^{j\;\phi}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$where e^(jϕ) is a cophasing factor between the two polarizations thatmay for instance be selected from a QPSK alphabet ϕϵ{0,π/2,π,3π/2}.

A corresponding precoder vector for a two-dimensional uniform planararray (UPA) with N₁×N₂ antennas can be created by taking the Kroneckerproduct of two precoder vectors as w_(2D)(l,m)=w_(1D)(l,N₁,O₁)⊗w_(1D)(m,N₂,O₂), where O₂ is an integeroversampling factor in the N₂ dimension. Each precoder w_(2D)(l,m) formsa DFT beam; all the precoders {w_(2D)(l,m), l=0, . . . , N₁O₁−1; m=0, .. . , N₂O₂−1} form a grid of DFT beams. An example is shown in FIG. 9Awhere (N₁, N₂)=(4,2) and (O₁, O₂)=(4,4). Throughout the followingsections, the terms ‘DFT beams’ and ‘DFT precoders’ are usedinterchangeably.

More generally, a beam with an index pair (l,m) can be identified by thedirection in which the greatest energy is transmitted when precodingweights w_(2D)(l,m) are used in the transmission. Also, a magnitudetaper can be used with DFT beams to lower the beam's sidelobes. A 1D DFTprecoder along N₁ and N₂ dimensions with magnitude tapering can beexpressed as:

${{w_{1D}\left( {l,N_{1},O_{1},\beta} \right)} = {\frac{1}{\sqrt{N_{1}}}\begin{bmatrix}{\beta_{0}e^{j\; 2{\pi \cdot 0 \cdot \frac{l}{O_{1}N_{1}}}}} \\{\beta_{1}e^{j\; 2{\pi \cdot 1 \cdot \frac{l}{O_{1}N_{1}}}}} \\\vdots \\{\beta_{N_{1} - 1}e^{j\; 2{\pi \cdot {({N_{1} - 1})} \cdot \frac{l}{O_{1}N_{1}}}}}\end{bmatrix}}},{{w_{1D}\left( {m,N_{2},O_{2},\gamma} \right)} = {\frac{1}{\sqrt{N_{2}}}\begin{bmatrix}{\gamma_{0}e^{j\; 2{\pi \cdot 0 \cdot \frac{m}{O_{2}N_{2}}}}} \\{\gamma_{1}e^{j\; 2{\pi \cdot 1 \cdot \frac{m}{O_{2}N_{2}}}}} \\\vdots \\{\gamma_{N_{2} - 1}e^{j\; 2{\pi \cdot {({N_{2} - 1})} \cdot \frac{m}{O_{2}N_{2}}}}}\end{bmatrix}}}$where 0<β_(i),γ_(k)≤1 (i=0, 1, . . . , N₁−1; k=0, 1, . . . , N₂−1) is anamplitude scaling factor. β_(i)=1,γ_(k)=1 (i=0, 1, . . . , N₁−1; k=0, 1,. . . , N₂−1) corresponds to no tapering. DFT beams (with or without amagnitude taper) have a linear phase shift between elements along eachof the two dimensions. Without loss of generality, it can be assumedthat the elements of w(l,m) are ordered according tow(l,m)=w_(1D)(l,N₁,O₁,β)⊗w_(1D)(m,N₂,O₂,γ) such that adjacent elementscorrespond to adjacent antenna elements along dimension N₂, and elementsof w(l,m) spaced N₂ apart correspond to adjacent antenna elements alongdimension N₁. Then the phase shift between two elements w_(s) ₁ (l,m)and w_(s) ₂ (l,m) of w(l,m) can be expressed as:

${w_{S_{2}}\left( {l,m} \right)} = {{w_{S_{1}}\left( {l,m} \right)} \cdot \left( \frac{\alpha_{S_{2}}}{\alpha_{S_{1}}} \right) \cdot e^{j\; 2\;{\pi{({{{({k_{1} - i_{1}})}{\Delta\;}_{1}} + {{({k_{2} - i_{2}})}{\Delta\;}_{2}}})}}}}$where s₁=i₁N₂+i₂ and s₂=k₁N₂+k₂ (with 0≤i₂<N₂, 0≤i₁<N₁, 0≤k₂<N₂, and0≤k₁<N₁) are integers identifying two entries of the beam w(l,m) so that(i₁, i₂) indicates to a first entry of beam w(l,m) that is mapped to afirst antenna element (or port) and (k₁, k₂) indicates to a second entryof beam w(l,m) that is mapped to a second antenna element (or port).

α_(s) ₁ =β_(i) ₁ γ_(i) ₂ and α_(s) ₂ =β_(k) ₁ γ_(k) ₂ are real numbers.α_(i)≠1 (i=s₁, s₂) if magnitude tapering is used; otherwise α_(i)=1.

$\Delta_{1} = \frac{l}{O_{1}N_{1}}$is a phase shift corresponding to a direction along an axis, e.g. thehorizontal axis (‘azimuth’).

$\Delta_{2} = \frac{m}{O_{2}N_{2}}$is a phase shift corresponding to direction along an axis, e.g. thevertical axis (‘elevation’).

Therefore a k^(th) beam d(k) formed with precoder w(l_(k),m_(k)) canalso be referred to by the corresponding precoder w(l_(k),m_(k)), i.e.d(k)=w(l_(k),m_(k)). Thus a beam d(k) can be described as a set ofcomplex numbers, each element of the set being characterized by at leastone complex phase shift such that an element of the beam is related toany other element of the beam where

d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))) = d_(i)(k)α_(i, n)(e^(j 2π Δ_(1, k)))^(p)(e^(j 2π Δ_(2, k)))^(q),where d_(i)(k) is the i^(th) element of a beam d(k), α_(i,n) is a realnumber corresponding to the i^(th) and n^(th) elements of the beam d(k);p and q are integers; and Δ_(1,k) and Δ_(2,k) are real numberscorresponding to a beam with index pair (l_(k),m_(k)) that determine thecomplex phase shifts e^(j2πΔ) ^(1,k) and e^(j2πα) ^(2,k) , respectively.Index pair (l_(k),m_(k)) corresponds to a direction of arrival ordeparture of a plane wave when beam d(k) is used for transmission orreception in a UPA or ULA. A beam d(k) can be identified with a singleindex k where=l_(k)+N₁O₁m_(k), i.e, along vertical or N₂ dimensionfirst, or alternatively k=N₂O₂l_(k)+m_(k), i.e. along horizontal or N₁dimension first.

Extending the precoder for a dual-polarized ULA may then be done as:

$\begin{matrix}{{w_{{2D},{DP}}\left( {l,m,\phi} \right)} = {{\begin{bmatrix}1 \\e^{j\;\phi}\end{bmatrix} \otimes {w_{2D}\left( {l, m} \right)}} = {\left\lbrack \begin{matrix}{w_{2D}\left( {l,m} \right)} \\{e^{j\;\phi}{w_{2D}\left( {l,m} \right)}}\end{matrix} \right\rbrack = {\left\lbrack \begin{matrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{matrix} \right\rbrack\left\lbrack \begin{matrix}1 \\e^{j\;\phi}\end{matrix} \right\rbrack}}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

A precoder matrix W_(2D,DP) for multi-layer transmission may be createdby appending columns of DFT precoder vectors as:W _(2D,DP) ^((R))=[w _(2D,DP)(l ₁ ,m ₁,ϕ₁)w _(2D,DP)(l ₂ ,m ₂,ϕ₂) . . .w _(2D,DP)(l _(R) ,m _(R),ϕ_(R))]where R is the number of transmission layers, i.e. the transmissionrank. In a special case for a rank-2 DFT precoder, m₁=m₂=m and l₁=l₂=l,we have:

$\begin{matrix}{{W_{{2D},{DP}}^{(2)}\left( {l,m,\phi_{1},\phi_{2}} \right)} = {\left\lbrack {{w_{{2D},{DP}}\left( {l,m,\phi_{1}} \right)}\mspace{14mu}{w_{{2D},{DP}}\left( {l,m,\phi_{2}} \right)}} \right\rbrack = {\left\lbrack \begin{matrix}{w_{2D}\left( {l,m} \right)} & 0 \\0 & {w_{2D}\left( {l,m} \right)}\end{matrix} \right\rbrack\begin{bmatrix}1 & 1 \\e^{j\;\phi_{1}} & e^{j\;\phi_{2}}\end{bmatrix}}}} & {{Equation}\mspace{14mu} 7}\end{matrix}$

For each rank, all the precoder candidates form a ‘precoder codebook’ ora ‘codebook’. A UE can first determine the rank of the estimateddownlink wideband channel based on CSI-RS. After the rank is identified,for each subband the UE then searches through all the precodercandidates in a codebook for the determined rank to find the bestprecoder for the subband. For example, in case of rank=1, the UE wouldsearch through w_(2D,DP)(k,l,ϕ) for all the possible (k,l,ϕ) values. Incase of rank=2, the UE would search through W_(2D,DP) ⁽²⁾ (k,l,ϕ₁,ϕ₂)for all the possible (k,l,ϕ₁,ϕ₂) values.

With multi-user MIMO (MU-MIMO), two or more users in the same cell areco-scheduled on the same time-frequency resource. That is, two or moreindependent data streams are transmitted to different UEs at the sametime, and the spatial domain is used to separate the respective streams.By transmitting several streams simultaneously, the capacity of thesystem can be increased. This, however, comes at the cost of reducingthe SINR per stream, as the power has to be shared between streams andthe streams will cause interference to each-other.

When increasing the antenna array size, the increased beamforming gainwill lead to higher SINR, however, as the user throughput depends onlylogarithmically on the SINR (for large SINRs), it is instead beneficialto trade the gains in SINR for a multiplexing gain, which increaseslinearly with the number of multiplexed users.

Accurate CSI is required in order to perform appropriate nullformingbetween coscheduled users. In the current LTE Rel. 13 standard, nospecial CSI mode for MU-MIMO exists and thus, MU-MIMO scheduling andprecoder construction has to be based on the existing CSI reportingdesigned for single-user MIMO (that is, a PMI indicating a DFT-basedprecoder, a RI and a CQI). This may prove quite challenging for MU-MIMO,as the reported precoder only contains information about the strongestchannel direction for a user and may thus not contain enough informationto do proper nullforming, which may lead to a large amount ofinterference between co-scheduled users, reducing the benefit ofMU-MIMO.

The DFT-based precoders discussed above and used in LTE Rel-13 calculatecophasing across pairs of (typically differently polarized) ports. Ifmore than one beam d(k) is used in CSI reporting, beams are not combinedwith the cophasing, but port pairs associated with a selected beam arecophased. Consequently, such DFT-based precoders can be considered as‘single beam’ precoders. Multi-beam precoders are therefore anextension, where cophasing is applied across beams as well as portpairs. Herein, we describe one such codebook. While the multi-beamcodebook is described with two dimensions of the codebook relating tohorizontal and vertical dimensions for concreteness, the codebook isequally applicable to a general case where the first or second dimensionrelates to horizontal or vertical antenna ports, as described above.

D_(N) is defined as a size N×N DFT matrix, i.e., the elements of D_(N)are defined as

$\left\lbrack D_{N} \right\rbrack_{k,l} = {{\frac{1}{\sqrt{N}}{e^{\frac{j\; 2\pi\;{kl}}{N}}.{R_{N}(q)}}} = {{diag}\left( \left\lbrack {e^{j\; 2\;{\pi \cdot 0 \cdot \frac{q}{N}}}\mspace{14mu} e^{j\; 2\;{\pi \cdot 1 \cdot \frac{q}{N}}}\mspace{14mu}\ldots\mspace{14mu} e^{j\; 2\mspace{2mu}{\pi \cdot {({N - 1})} \cdot \frac{q}{N}}}} \right\rbrack \right)}}$is further defined to be a size N×N rotation matrix, defined for 0≤q<1.Multiplying D_(N) with R_(N)(q) from the left creates a rotated DFTmatrix with entries

$\left\lbrack {{R_{N}(q)}D_{N}} \right\rbrack_{k,l} = {\frac{1}{\sqrt{N}}{e^{\frac{j\; 2\pi\;{k{({l + q})}}}{N}}.}}$The rotated DFT matrix R_(N)(q)D_(N)=[d₁ d₂ . . . d_(N)] consists ofnormalized orthogonal column vectors {d_(i)}_(i=1) ^(N) whichfurthermore span the vector space

^(N). That is, the columns of R_(N)(q)D_(N), for any q, is anorthonormal basis of

^(N).

In some embodiments, a codebook design is created by extending the(rotated) DFT matrices that were appropriate transforms for asingle-polarized ULA as discussed above to also fit the more generalcase of dual-polarized 2D UPAs.

A rotated 2D DFT matrix is defined as D_(N) _(V) _(,N) _(H)(q_(V),q_(H))=(R_(N) _(H) (q_(H))D_(N) _(H) )⊗(R_(N) _(V) (q_(V))D_(N)_(V) )=[d₁ d₂ . . . d_(N) _(V) _(N) _(H) ]. The columns {d_(i)}_(i=1)^(N) ^(DP) of D_(N) _(V) _(,N) _(H) (q_(V),q_(H)) constitutes anorthonormal basis of the vector space

^(N) ^(V) ^(N) ^(H) . Such a column d_(i) is henceforth denoted a (DFT)beam.

A dual-polarized beam space transformation matrix suitable for a UPA iscreated where the upper left and lower right elements correspond to thetwo polarizations:

${B_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)} = {{I_{2} \otimes {D_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)}} = {\begin{bmatrix}{D_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)} & 0 \\0 & {D_{N_{V},N_{H}}\left( {q_{V},q_{H}} \right)}\end{bmatrix} = \mspace{85mu}{\left\lbrack \begin{matrix}d_{1} & d_{2} & \ldots & d_{N_{V}N_{H}} & 0 & 0 & \ldots & 0 \\0 & 0 & \ldots & 0 & d_{1} & d_{2} & \ldots & d_{N_{V}N_{H}}\end{matrix} \right\rbrack = {\left\lbrack {b_{1}\mspace{14mu} b_{2}\mspace{14mu}\ldots\mspace{14mu} b_{2N_{V}}N_{H}} \right\rbrack.}}}}$

The columns {b_(i)}_(i=1) ^(2N) ^(V) ^(N) ^(H) of B_(N) _(V) _(,N) _(H)(q_(V),q_(H)) constitute an orthonormal basis of the vector space

^(2N) ^(V) ^(N) ^(H) . Such a column b_(i) is henceforth denoted as asingle-polarized beam (SP-beam) as it is constructed by a beam dtransmitted on a single polarization

$\left( {{i.e.\mspace{14mu} b} = {{\begin{bmatrix}d \\0\end{bmatrix}\mspace{14mu}{or}\mspace{14mu} b} = \begin{bmatrix}0 \\d\end{bmatrix}}} \right).$The notation dual-polarized beam is also introduced to refer to a beamtransmitted on both polarizations (which are combined with apolarization cophasing factor e^(jα),

${i.e.\mspace{14mu} b_{DP}} = {\begin{bmatrix}d \\{e^{j\;\alpha}d}\end{bmatrix}{\text{)}.}}$

Utilizing the assumption that the channel is somewhat sparse, much ofthe channel energy is captured by only selecting a column subset ofB_(N) _(V) _(,N) _(H) (q_(V),q_(H)) that is, it is sufficient todescribe a couple of the SP-beams, which keeps down the feedbackoverhead. Therefore, selecting a column subset I_(S) consisting ofN_(SP) columns of B_(N) _(V) _(,N) _(H) (q_(V),q_(H)), creates a reducedbeam space transformation matrix B_(I) _(S) =[b_(I) _(S) ₍₁₎ b_(I) _(S)₍₂₎ . . . b_(I) _(S) _((N) _(SP) ₎], eg., selecting column numbersI_(S)=[1 5 10 25] creates the reduced beam space transformation matrixB_(I) _(S) =[b₁ b₅ b₁₀ b₂₅].

A general precoder structure for precoding of a single layer is:

$w = {{B_{I_{S}}\begin{bmatrix}c_{1} \\c_{2} \\\vdots \\c_{N_{SP}}\end{bmatrix}} = {{\left\lbrack {b_{I_{S}{(1)}}\mspace{14mu} b_{I_{S}{(2)}}\mspace{14mu}\ldots\mspace{14mu} b_{I_{S}{(N_{SP})}}} \right\rbrack\begin{bmatrix}c_{1} \\c_{2} \\\vdots \\c_{N_{SP}}\end{bmatrix}} = {\sum\limits_{i = 1}^{N_{SP}}{c_{i}{b_{I_{S}{(i)}}.}}}}}$where {c_(i)}_(i=1) ^(N) ^(SP) are complex beam cophasing coefficients.

The precoder w in the equation above can be described as a linearcombination of beams constructed by cophasing a k^(th) beam b_(k) withcophasing coefficient c_(k). Such a beam cophasing coefficient is ascalar complex number that adjusts at least the phase of a beam relativeto other beams according to c_(k)b_(k). When a beam cophasingcoefficient only adjusts relative phase, it is a unit magnitude complexnumber. It is in general desirable to also adjust the relative gain ofbeams, in which case the beam cophasing coefficient is not unitmagnitude.

A more refined multi-beam precoder structure is achieved by separatingthe complex coefficients in a power (or amplitude) and a phase part as:

$\begin{matrix}{w = {{B_{I_{S}}\begin{bmatrix}c_{1} \\c_{2} \\\vdots \\c_{N_{SP}}\end{bmatrix}} = {B_{I_{S}}\begin{bmatrix}{\sqrt{p_{1}}e^{j\;\alpha_{1}}} \\{\sqrt{p_{2}}e^{j\;\alpha_{2}}} \\\vdots \\{\sqrt{p_{N_{SP}}}e^{j\;\alpha_{N_{SP}}}}\end{bmatrix}}}} \\{= {{B_{I_{S}}\begin{bmatrix}\sqrt{p_{1}} & 0 & \; & \; \\0 & \sqrt{p_{2}} & \; & {\ddots\;} \\\; & \; & \ddots & 0 \\\; & {\ddots\;} & 0 & \sqrt{p_{N_{SP}}}\end{bmatrix}}\begin{bmatrix}e^{j\;\alpha_{1}} \\e^{j\;\alpha_{2}} \\\vdots \\e^{j\;\alpha_{N_{SP}}}\end{bmatrix}}} \\{= {B_{I_{S}}{\sqrt{P}\begin{bmatrix}e^{j\;\alpha_{1}} \\e^{j\;\alpha_{2}} \\\vdots \\e^{j\;\alpha_{N_{SP}}}\end{bmatrix}}}}\end{matrix}$

As multiplying the precoder vector w with a complex constant C does notchange its beamforming properties (as only the phase and amplituderelative to the other single-polarized beams is of importance), one maywithout loss of generality assume that the coefficients corresponding toe.g. SP-beam 1 is fixed to p₁=1 and e^(jα) ¹ =1, so that parameters forone less beam needs to be signaled from the UE to the base station.Furthermore, the precoder may be further assumed to be multiplied with anormalization factor, so that, e.g., a sum power constraint isfulfilled, i.e. that ∥w∥²=1. Any such normalization factor is omittedfrom the equations herein for clarity.

In some cases, the possible choices of columns of B_(N) _(V) _(,N) _(H)(q_(V),q_(H)) are restricted so that if column i=i₀ is chosen, so iscolumn i=i₀+N_(V)N_(H). That is, if an SP-beam corresponding to acertain beam maimed to the first polarization is chosen, e.g.

${b_{i_{0}} = \begin{bmatrix}d_{i_{0}} \\0\end{bmatrix}},$this would imply that the SP-beam

$b_{i_{0} + {N_{V}N_{H}}} = \begin{bmatrix}0 \\d_{i_{0}}\end{bmatrix}$is chosen as well. That is, the SP-beam corresponding to the saidcertain beam mapped to the second polarization is chosen as well. Thiswould reduce the feedback overhead as only N_(DP)=N_(SP)/2 columns ofB_(N) _(V) _(,N) _(H) (q_(V),q_(H)) would have to be selected andsignaled back to the base station. In other words, the column selectionis done on a beam (or DP-beam) level rather than an SP-beam level. If acertain beam is strong on one of the polarizations it would typicallyimply that the beam would be strong on the other polarization as well,at least in a wideband sense, so the loss of restricting the columnselection in this way would not significantly decrease the performance.In the following discussion, the use of DP-beams is generally assumed(unless stated otherwise).

In some cases, the multi-beam precoder is factorized into two or morefactors that are selected with different frequency-granularity, in orderto reduce the feedback overhead. In such cases, the SP-beam selection(i.e. the choice of matrix B_(I) _(S) ) and the relative SP-beampowers/amplitudes (i.e. the choice of matrix √{square root over (P)})are selected with a certain frequency-granularity while the SP-beamphases (i.e. the choice of matrix

$\begin{bmatrix}e^{j\;\alpha_{1}} \\e^{j\;\alpha_{2}} \\\vdots \\e^{j\;\alpha_{N_{SP}}}\end{bmatrix}\text{)}$are selected with another certain frequency-granularity. In one suchcase, the certain frequency-granularity corresponds to a widebandselection (that is, one selection for the entire bandwidth) while thesaid another certain frequency-granularity corresponds to a per-subbandselection (that is, the carrier bandwidth is split into a number ofsubbands, typically consisting of 1-10 PRBs, and a separate selection isdone for each subband).

In a typical case, the multi-beam precoder vector is factorized asw=W₁W₂, where W₁ is selected with a certain frequency-granularity and W₂is selected another certain frequency-granularity. The precoder vectormay then be expressed as

${\underset{= W_{1}}{\underset{︸}{w = {B_{I_{S}}\sqrt{P}}}}\underset{= W_{2}}{\underset{︸}{\begin{bmatrix}e^{j\;\alpha_{1}} \\e^{j\;\alpha_{2}} \\\vdots \\e^{j\;\alpha_{N_{SP}}}\end{bmatrix}}}} = {W_{1}{W_{2}.}}$Using this notation, if the said certain frequency-granularitycorresponds to a wideband selection of W₁ and the said another certainfrequency-granularity corresponds to a per-subband selection of W₂, theprecoder vector for subband l may be expressed as w_(l)=W₁W₂(l). Thatis, only W₂ is a function of the subband index l.

What needs to be fed back by the UE to the eNodeB is thus:

the chosen columns of B_(N) _(V) _(,N) _(H) (q_(V),q_(H)), i.e., theN_(SP) single-polarized beams. This requires at mostN_(SP)·log₂(2N_(V)N_(H)) bits.

the vertical and horizontal DFT basis rotation factors q_(V) and q_(H).For instance, the

${{q(i)} = \frac{i}{Q}},{i = 0},1,\ldots\mspace{14mu},{Q - 1},$for some value of Q. The corresponding overhead would then be 2·log₂ Qbits.

the (relative) power levels {p₂, p₃, . . . , p_(N) _(SP) } of theSP-beams. If L is the number of possible discrete power levels,(N_(SP)−1)·log₂ L is needed to feedback the SP-beam power levels.

the cophasing factors {e^(jα) ² , e^(jα) ³ , . . . ,

e^(j α_(N_(SP)))}of the SP-beams. For instance,

${{\alpha(k)} = \frac{2\pi\; k}{K}},{k = 0},1,{{\ldots\mspace{14mu} K} - 1},$for some value of K. The corresponding overhead would be,(2N_(DP)−1)·log₂ K bits per rank per W₂(l) report.

Recently, 3GPP has agreed to the following working assumption used todevelop physical layer specifications for Rel-14 advanced CSI based onmulti-beam precoders. Note that the term ‘beam combining coefficient’ isused for the cophasing factors c_(r,l,i) here, although the cophasingfactors can combine elements with different polarizations as well asdifferent beams.

Precoders are to be normalized in the equations below. FIG. 9Billustrates an example for W1 beam selection, W1 beam power, and W2determination according to some embodiments of the present disclosure.

${W_{1} = \begin{bmatrix}B & 0 \\0 & B\end{bmatrix}},{B = \left\lbrack {{p_{0}b_{k_{1}^{(0)},k_{2}^{(0)}}},\ldots\;,\ {p_{L - 1}b_{k_{1}^{({L - 1})},k_{2}^{({L - 1})}}}} \right\rbrack}$${{{{For}\mspace{14mu}{rank}\mspace{14mu} 1}:W} = {\begin{bmatrix}{\overset{˜}{w}}_{0,0} \\{\overset{˜}{w}}_{1,1}\end{bmatrix} = {W_{1}W_{2}}}},{{{and}\mspace{14mu} W_{2}} = \begin{bmatrix}c_{0,0} \\c_{1,0}\end{bmatrix}}$${{{{For}\mspace{14mu}{rank}\mspace{14mu} 2}:W} = {\begin{bmatrix}{\overset{˜}{w}}_{0,0} & {\overset{˜}{w}}_{0,1} \\{\overset{˜}{w}}_{1,0} & {\overset{˜}{w}}_{1,1}\end{bmatrix} = {W_{1}W_{2}}}},\;{{{and}\mspace{14mu} W_{2}} = \begin{bmatrix}c_{0,0} & c_{0,1} \\c_{1,0} & c_{1,1}\end{bmatrix}}$c_(r, l) = [c_(r, l, 0), … , c_(r, l, L − 1)]^(T), r = 0, 1, l = 0, 1${{{\overset{˜}{w}}_{r,l} = {\sum\limits_{i = 0}^{L - 1}\;{b_{k_{1}^{(L)}k_{2}^{(L)}} \cdot p_{i} \cdot c_{r,l,i}}}};{r = 0}},1,{l = 0},1$ L = 2 is the number of beams  b_(k) ₁ _(,k) ₂ is a 2D DFT beam fromoversampled grid  k₁ = 0,1, . . . , N₁O₁ − 1  k₂ = 0,1, . . . , N₂O₂ − 1 O ≤ ρ_(i) ≤ 1 beam power scaling factor for beam i  c_(r,l,i) beamcombining coefficient for beam i and on polarization r and layer l  w1overhead for N₁ = N₂ = 4  Indicate leading beam: W1 W2  [log₂(N₁N₂O₁O₂)]= [log2(16N₁N₂)] = 8 bits Rank (bits) (bits)${{Indicate}\mspace{14mu}{second}\mspace{14mu}{{beam}:\left\lceil \begin{pmatrix}7 \\1\end{pmatrix} \right\rceil}} = {3\mspace{14mu}{bits}}$Relative  power  of  weaker  beam : 2  bits 1 2 13 13 6 12

Feedback on PUSCH is supported and feedback on PUCCH is supported.Because feedback on PUCCH is to be supported, and since indications ofW₁ and W₂ are (at least in some cases) larger than can be supported onPUCCH Format 2, the feedback for W₁ and/or W₂ must be modified whenreporting on PUCCH Format 2 is configured.

FIGS. 10A through 13A illustrate procedures for reporting CSI feedbackon a physical channel according to some embodiments of the presentdisclosure.

FIG. 10A illustrates a procedure by which the second node 14 reports CSIfeedback to the first node 12 on a physical channel (step 100A).According to some embodiments, the CSI feedback is rich CSI feedback. Asused herein, rich CSI refers to CSI that conveys more information thantraditional CSI. For example, rich CSI may be a CSI for LTE Advanced orfor NR Type 2. Additional examples and description are included below.According to some embodiments, the reporting of CSI feedback is with asmall payload. Also, as used herein, a small payload is a payload thatincludes less total bits than what would usually need to be sent inother applications. For example, an application for advanced CSI is totransmit subband PMI, using a number of bits per subband (consideredsubstantial). Compared to this application, according to some disclosedembodiments, the payload is constrained when there is a need to transmitwideband PMI and further subsample the PMI so that it fits the feedbackchannel. In such case, a small payload is a payload small enough to fitthe feedback channel or smaller. This may be accomplished in manydifferent ways, some of which are discussed below. Specifically, asshown in FIG. 11A, the second node 14 identifies a subset of codebookentries from an advanced CSI codebook of coefficients (step 200A). Then,the second node 14 selects a codebook entry from the subset (202A). Anindex of the selected codebook entry is reported to the first node 12(step 204A). In this way, the constraints of the physical channel withthe small payload are met, even when sending rich CSI.

FIG. 12A illustrates a procedure by which the second node 14 reports arank indicator and a beam count indicator in a first transmission (step300A) and reports a cophasing indicator in a second transmission (step302A). In some embodiments, both of these transmissions are sent on thesame uplink control channel. In some embodiments, these transmissionsare sent on a channel that is acting as a control channel. In someembodiments, the second node 14 determines a number of beams L used toconstruct the multi-beam CSI report (step 304A). The second node 14 thendetermines a beam indicator for an l^(th) beam, the beam indicatoridentifying the index of a beam of the multi-beam CSI report if L is atleast l, and otherwise identifying that L is less than l (step 306A).

FIG. 13A illustrates a procedure by which the second node 14 reports CSIcorresponding to a first number of beams if the CSI corresponds to afirst rank (step 400A) and reports CSI corresponding to a second numberof beams if the CSI corresponds to a second rank (step 402A).

FIGS. 10B-13B, are figures illustrating analogous operation at areceiving side such as first node 12.

In LTE Rel-13 Class A codebook based periodic CSI feedback is carried onPUCCH Format 2 over at least three transmissions, i.e.

-   -   1^(St) transmission: RI    -   2^(nd) transmission: W₁    -   3^(rd) transmission: W₂ and CQI

For each transmission, up to 11 bits can be transmitted. A primary aimis to have also three transmissions for advanced CSI feedback over PUCCHFormat 2.

As it is possible to multiplex periodic CSI feedback over several PUCCHtransmissions, the individual components comprising the PMI feedbackindicating the selection of W₁ and W₂ are reiterated.

The reporting of W₁ can be split up into separate components, as wasfurther elaborated in the background:

-   -   leading beams selection: log₂(N_(V)·N_(H))=4 bits, in the worst        case of 2N_(V)N_(H)=32 antenna ports    -   beam rotations: log₂(Q_(H)·Q_(V))=log₂(4·4)=4 bits    -   second beam selection: [log₂(7)]=3 bits    -   beam relative power: 2 bits

Although the codebook defines precoders as linear combinations of L=2beams (or NDP beams using the notation in the description of multi-beamprecoders above), it is possible to set the relative beam power of thesecond beam to zero, resulting in an effective precoder comprising onlyL=1 beam. In such a case, precoder components describing a second beamdo not need to be known to construct the precoder and correspondingly,no signaling indicating said precoder components are needed.

Thus, the reporting of the W₂ matrix uses (2L−1) N_(p) r bits persubband, where L is the number of beams, N_(p) is the number of phasebits per element of W₂ (or log₂ K bits using the notation of themultibeam precoder discussion above), and r is the rank. Since a QPSKconstellation is used, N_(p)=2 and the number of bits for W₂ per subbandfor L=1 and L=2 are summarized in Table 2:

TABLE 2 W₂ beam cophasing overhead (per subband) Beams (L) Rank (r) 1 21 2 bits  6 bits 2 4 bits 12 bits

Since it may be beneficial to report W₂ together with CQI in a PUCCHtransmission, for PUCCH Format 2, the total payload can be no more than11 bits. Because CQI occupies 4 and 7 bits for 1 and 2 codewordsrespectively, W₂ can occupy no more than 7 or 4 bits for rank 1 or 2 (asrank 1 uses 1 codeword while rank 2 uses 2 codewords in LTE). Therefore,wideband W₂ PMI for rank 1 can fit on PUCCH Format 2 withoutsubsampling, whereas subsampling 12 bits to 4 bits is needed for rank 2,for L=2. This constitutes a substantial subsampling.

Given the above constraints, three different payload sizes (2, 4, or 6)may be used for W₂ on PUCCH Format 2. eNB must be aware of the number ofbeams and the rank used to compute W₂ if the payload size varies. Sincein Rel-13, eNB determines the size of the CQI field based on the RI,that principle can be reused to determine the rank used to compute W₂.If the beam power field is encoded independently of W₂, then the numberof beams used to determine W₂ could also be determined by eNB from thereported beam power field.

The table below shows the W₂ payload sizes.

TABLE 3 W₂ payload alternatives Alternative W₂ + CQI payload One or Twobeams, ranks 1 & 2 Rank 1: {2 or 6} + 4 bits = 6 or 10 bits Rank 2: 4 +7 bits = 11 bits

The rich W₂ CSI feedback in LTE Rel-14 implements a scalar quantizationof beam and polarization cophasing for each layer, where the W₂ matrixfor rank 2 may be expressed as:

$W_{2} = \begin{bmatrix}1 & 1 \\c_{10} & c_{11} \\c_{20} & c_{21} \\c_{30} & c_{31}\end{bmatrix}$where each c_(i,j)ϵ{1,j,−1,−j}, i.e. each element may be independentlychosen from a QPSK constellation. To further clarify, c_(1j) denotes arelative phase of the first and second beam on a first polarization,c_(2j) denotes a relative phase between the two polarizations of thefirst beam, and c_(3j) denotes the relative phase of the first beam onthe first polarization and the second beam on the second polarization.Since scalar quantization is used, W₂ may be parametrized using the D=6dimensional vector c=[c₁₀ c₂₀ c₃₀ c₁₁ c₃₁]^(T) and may thus beconsidered to have six degrees of freedom, resulting in S=N_(p)^(D)=4⁶=4096 possible states, represented by 12 bits. The W₂ codebookmay thus be indexed with k=0, 1, . . . , S−1.

One approach to subsampling the W₂ codebook is to merely subsample theindex k so that only every X^(th) index may be chosen and instead reportthe index

${\overset{\sim}{k} = 0},1,\ldots\mspace{14mu},{\frac{S}{X} - 1},$where k=X·{tilde over (k)}. However, such a subsampling does not utilizethe structure of the codebook and may provide low CSI granularity.

Another approach to subsampling the codebook is to lower theconstellation alphabet size, so that for instance c_(i,j)ϵ{1,−1} and aBinary Phase Shift Keying (BPSK) constellation is used. In our example,though, this would still require 6 bits of feedback overhead whichovershoots the target of 4 bits for rank 2. Note that since the BPSKconstellation points are comprised in the QPSK constellation, loweringthe constellation alphabet size in such a manner constitutes a codebooksubsampling since all the resulting precoders in the subsampled codebookare comprised in the non-subsampled codebook.

However, in order to further reduce the feedback overhead, a method ofrich CSI W₂ codebook subsampling is presented herein. The method worksby parametrizing the W₂ codebook using a smaller number of parameters Mthan the required D parameters to span the entire codebook. That is, theprecoders in the subsampled W₂ may be generated from a size-M vector{tilde over (c)}=[{tilde over (c)}₀ . . . {tilde over (c)}_(M-1)]^(T)and a fixed mapping from {tilde over (c)} to precoder matrix.

As an illustrative embodiment, consider M=1 so that {tilde over(c)}={tilde over (c)}₀. The subsampled precoder codebook may then begenerated as, for instance,

${\overset{\sim}{W}}_{2} = \begin{bmatrix}1 & 1 \\{\overset{\sim}{c}}_{0} & {- {\overset{\sim}{c}}_{0}} \\{- {\overset{\sim}{c}}_{0}} & {\overset{\sim}{c}}_{0} \\{\overset{\sim}{c}}_{0} & {- {\overset{\sim}{c}}_{0}}\end{bmatrix}$

If {tilde over (c)}₀ϵ{1,j,−1,−j}, there are thus 4¹=4 possible W₂matrices in the subsampled codebook. Note that all possible {tilde over(W)}₂ are comprised in the non-subsampled codebook, and {tilde over(W)}₂ thus constitutes a codebook subsampling and not a new, separatecodebook. For this to hold true, it is required that each elementc_(i,j) of the precoder matrices in the subsampled codebook belongs tothe same constellation as the non-subsampled codebook (e.g. QPSK{1,j,−1,−j}). As Phase Shift Keying (PSK) constellations are closedunder multiplication, one may thus construct c_(i,j) by multiplying anarbitrary number of PSK symbols. Thus, if the elements of {tilde over(c)} are from the same constellation as the elements in thenon-subsampled codebook, and the elements in {tilde over (W)}₂ areformed by multiplying elements of {tilde over (c)} or other PSK symbols(note that “−1” is a PSK symbol), {tilde over (W)}₂ is ensured to becomprised in the non-subsampled codebook. Based on these rules forgenerating codebook subsamplings according to the method, {tilde over(W)}₂-matrices that give a good tradeoff between performance andfeedback overhead may be designed.

In some embodiments, a codebook subsampling is generated utilizing twoproperties:

-   -   Phase offset between beams are (partly) due to differences in        propagation delay and so may be similar on both polarizations    -   The precoding on different layers are often chosen to be        mutually orthogonal

The first property suggests that the ratios c_(1,j)/1 andc_(3,j)/c_(2,j) may be similar in certain propagation conditions. Thiscan be utilized in subsampling design so that the precoding of a singlelayer may be expressed as:

$\begin{bmatrix}1 \\c \\\varphi \\{c\;\varphi}\end{bmatrix}\quad$where c is a beam cophasing coefficient and φ is a polarizationcophasing coefficient, which both are QPSK symbols. Thus, with thisdesign, the ratios

${\frac{c_{1,j}}{1} = {\frac{c_{3,j}}{c_{2,j}} = c}},$fulfilling the first desired property.

To fulfill the second property, the second layer may be designed to beorthogonal to the first layer so that {tilde over (W)}₂ ^(H){tilde over(W)}₂=σ·I, where I is the identity matrix (a matrix of all zeroes excepton the diagonal, which contains all ones), and σ is a non-negativescalar. This may be achieved by copying the coefficients for the firstlayer but negating the entries corresponding to the second polarizationas:

${\overset{\sim}{W}}_{2} = {\begin{bmatrix}1 & 1 \\c & c \\\varphi & {- \varphi} \\{c\;\varphi} & {{- c}\;\varphi}\end{bmatrix}\quad}$

Thus, with this subsampling design, both desired properties arefulfilled. Furthermore, the subsampled codebook is generated from {tildeover (c)}=[cϕ]^(T), i.e. using 2 parameters, where each element in{tilde over (c)} belongs to a QPSK constellation. Thus, 2+2=4 bits areneeded to indicate an element in the subsampled codebook, which meetsthe requirement on PUCCH feedback overhead for W₂.

In some embodiments, the property that layers are often chosen to bemutually orthogonal is not utilized in the subsampling design, as thisputs an unnecessary restriction on the channel quantization for somepropagation conditions. Instead, each layer is encoded independently.The previously mentioned first property is still utilized though, sothat a separate beam cophasing coefficient and polarization coefficientis used, resulting in a matrix design:

${\overset{\sim}{W}}_{2} = \begin{bmatrix}1 & 1 \\c_{0} & c_{1} \\\varphi_{0} & \varphi_{1} \\{c_{0}\;\varphi_{0}} & {c_{1}\;\varphi_{1}}\end{bmatrix}$

Thus, the subsampled codebook may be generated from 4 parameters {tildeover (c)}=[c₀ c₁ ϕ₀ ϕ₁]^(T) in this embodiment. To meet the requirementof a 4 bit W₂ report though, each parameter cannot be selected from aQPSK constellation, as this would require an 8 bit report. However, asthe BPSK constellation points are comprised within the QPSKconstellation, using a lower order constellation for the parameters willstill ensure that the {tilde over (W)}₂ constitutes a subsampledcodebook. Thus, if each parameter is selected from a BPSK constellation,the subsampled codebook may be reported with 4 bits and the requirementis met.

A UE assumes L=2 is used for reporting W₂ if rank=1 and L=1 if rank=2.In this case, there is no subsampling required for either rank=1 orrank=2 W₂, since 6 bits and 4 bits can be carried with CQI for rank 1and rank 2, respectively, as discussed above with respect to W₂ payloadalternatives. For rank=1, the full resolution of W₂ is preserved, andthe full size W₂ (6 bits in the case of the Rel-14 codebook) isreported. For rank=2, a single beam is used for W₂, which corresponds tothe W₂ with the non-subsampled multi-beam codebook, and so requires 4bits to signal W₂ using the Rel-14 codebook.

For PUCCH Format 2, the following design goals for consistency withRel-13 operation are identified:

-   -   1. All CSI reporting types must fit into 11 bits    -   2. At most 3 transmissions are needed to report RI, CQI, PMI,        and CRI.        -   a. RI is carried in one transmission        -   b. Wideband CQI with 4 or 7 bits can be used for 1 or 2            codeword transmission, respectively, and is carried in            another PUCCH transmission.        -   c. At least the beam index is carried in a third PUCCH            transmission.    -   3. Each transmission should be as useful as possible to the        eNodeB in the absence of the other transmissions.

Since the RI often needs to be decoded to determine the size of otherCSI fields, such as the CQI and the PMI, it is important that it bereceived reliably. Consequently, the RI should be multiplexed in a PUCCHtransmission with as few as possible other fields, while still providingthe needed CSI. Transmitting as little extra information as possiblemeans that fewer bits are present in the PUCCH carrying the RI, and soit is received more reliably at a given received SINR.

The beam power indication and the second beam index require 2 and 3bits, respectively. On the other hand, the first beam index requires atleast 4 bits (8 bits if the index includes the rotation, as is done inthe Rel-14 codebook agreement). Since the first beam index should bereported together with (or directly include) the beam rotation, these 8bits should be reported in one PUCCH transmission. Overall, then, thebeam power indication and the second beam index are reasonablecandidates to multiplex with RI, whereas the first beam index and/orbeam rotation are not.

If RI is multiplexed with the second beam index, then if Rel-13 PUCCHreporting timing is used, since RI (for example PUCCH reporting type 3or 7), is likely to be reported more slowly than wideband PMI (i.e.PUCCH reporting type 2a), the two beams would be reported at differentrates, which is undesirable, since they have the same basiccharacteristics and vary with propagation at the same rate in time. Thisunequal reporting rate will also likely degrade performance. Therefore,it does not seem desirable to report the second beam index with RI.

Reporting the beam power indication with RI makes intuitive sense, sincethe number of beams in the channel is similar to its rank, as the numberof beams identifies the number of parameters needed to approximate thechannel just as the rank does. Furthermore, the beam power indicationidentifies if precoder parameters for the second beam need to be known,and so can be considered a beam count indicator.

The beam power field (also ‘beam count indicator’) can be used toidentify the size of the W₂ cophasing indicator and the presence ofinformation identifying the second beam. If the beam power fieldcorresponding to the 2^(nd) beam indicates a non-zero value (forexample, 1, √{square root over (0.5)}, or √{square root over (0.25)}),then the CSI report corresponds to 2 beams. In this case, the secondbeam index is reported, and the size of a wideband cophasing indicatorW₂ reported on PUCCH will be 4 bits (with W₂ subsampling as discussedabove). If the beam power field indicates a zero value, then the secondbeam index is not reported, and the size of a wideband cophasingindicator W₂ reported on PUCCH will be 2 or 4 bits (also as discussedabove with respect to W₂ beam cophasing overhead per subband), dependingon if rank 1 or rank 2, respectively, is indicated by RI.

Therefore, in an embodiment, a rank indicator and a beam count indicatorare both transmitted in one transmission. The rank indicator identifiesthe rank used when computing the CSI feedback to which the rank relates.The beam count indicator identifies at least the number of beams usedwhen computing the CSI feedback, and may additionally indicate therelative power of beams identified in the CSI feedback. The rank andbeam count indicators may identify the size of a CSI feedback fieldtransmitted in a separate transmission, such as a cophasing indicator(W₂) or a beam index (W₁). With this embodiment, the advanced CSIfeedback can be carried on PUCCH Format 2 over at least threetransmissions, i.e.

-   -   1. 1st transmission: RI+beam power (or beam count indicator)    -   2. 2nd transmission: W₁ (first beam index+beam rotation+second        beam index)    -   3rd transmission: W₂ and CQI

Note that while the transmissions may be sequenced in time in the orderof their numbering, this is not required. Also, these may be sent ascompletely separate transmissions or as separate parts of the sametransmission.

In a related embodiment, a later transmission carries a CQI field and acophasing indicator field (W₂). The size of the cophasing indicatorfield is determined by at least a beam count indicator transmitted in anearlier transmission, and the size of the CQI field is determined by atleast an RI transmitted in the earlier transmission.

It may also be desirable to provide an alternative indication of thenumber of beams used in the multi-beam CSI report. This can allow thenumber of beams to be reported to eNB more often than when the number ofbeams is only provided in reports containing RI, since RI is generallyreported infrequently. In this case, a CSI report for the second(weaker) beam jointly identifies the number of beams and an index of thesecond beam. The particular codebook design used in 3GPP is well suitedto this, since the second beam index has 7 possible values, and so an8th value indicating if the second beam is present can fit in a 3 bitindicator.

Therefore, in an embodiment, a first transmission carries a beam indexthat is jointly encoded with an indication of if a second beam is notpresent where when the second beam is not present corresponds to a beampower of 0 for the second beam. Additionally, a second transmission maycarry a cophasing indicator field. The size of the cophasing indicatorfield is determined by at least the indication of if a second beam isnot present.

FIGS. 14 and 15 illustrate example embodiments of a second node 14 suchas a wireless device 14 according to some embodiments of the presentdisclosure. FIG. 14 is a schematic block diagram of the wireless device14 (e.g., a UE 14) according to some embodiments of the presentdisclosure. As illustrated, the wireless device 14 includes circuitry 18comprising one or more processors 20 (e.g., Central Processing Units(CPUs), Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs), and/or the like) and memory 22. Thewireless device 14 also includes one or more transceivers 24 eachincluding one or more transmitter 26 and one or more receivers 28coupled to one or more antennas 30. In some embodiments, thefunctionality of the wireless device 14 described above may be fully orpartially implemented in software that is, e.g., stored in the memory 22and executed by the processor(s) 20.

In some embodiments, a computer program including instructions which,when executed by at least one processor, causes the at least oneprocessor to carry out the functionality of the wireless device 14according to any of the embodiments described herein is provided. Insome embodiments, a carrier containing the aforementioned computerprogram product is provided. The carrier is one of an electronic signal,an optical signal, a radio signal, or a computer readable storage medium(e.g., a non-transitory computer readable medium such as memory).

FIG. 15 is a schematic block diagram of the wireless device 14 accordingto some other embodiments of the present disclosure. The wireless device14 includes one or more modules 32, each of which is implemented insoftware. The module(s) 32 provide the functionality of the wirelessdevice 14 (e.g., UE 14) described herein.

FIGS. 16 through 18 illustrate example embodiments of a radio networknode according to some embodiments of the present disclosure. FIG. 16 isa schematic block diagram of the node 12 according to some embodimentsof the present disclosure. Other types of network nodes may have similararchitectures (particularly with respect to including processor(s),memory, and a network interface). As illustrated, the radio access node12 includes a control system 34 that includes circuitry comprising oneor more processors 36 (e.g., CPUs, ASICs, FPGAs, and/or the like) andmemory 38. The control system 34 also includes a network interface 40.The radio access node 12 also includes one or more radio units 42 thateach include one or more transmitters 44 and one or more receivers 46coupled to one or more antennas 48. In some embodiments, thefunctionality of the radio access node 12 described above may be fullyor partially implemented in software that is, e.g., stored in the memory38 and executed by the processor(s) 36.

FIG. 17 is a schematic block diagram that illustrates a virtualizedembodiment of the radio access node 12 according to some embodiments ofthe present disclosure. Other types of network nodes may have similararchitectures (particularly with respect to including processor(s),memory, and a network interface).

As used herein, a “virtualized” radio access node 12 is a radio accessnode 12 in which at least a portion of the functionality of the radioaccess node 12 is implemented as a virtual component (e.g., via avirtual machine(s) executing on a physical processing node(s) in anetwork(s)). As illustrated, the radio access node 12 optionallyincludes the control system 34, as described with respect to FIG. 16.The radio access node 12 also includes the one or more radio units 42that each include the one or more transmitters 44 and the one or morereceivers 46 coupled to the one or more antennas 48, as described above.The control system 34 (if present) is connected to the radio unit(s) 42via, for example, an optical cable or the like. The control system 34(if present) is connected to one or more processing nodes 50 coupled toor included as part of a network(s) 52 via the network interface 40.Alternatively, if the control system 34 is not present, the one or moreradio units 42 are connected to the one or more processing nodes 50 viaa network interface(s). Each processing node 50 includes one or moreprocessors 54 (e.g., CPUs, ASICs, FPGAs, and/or the like), memory 56,and a network interface 58.

In this example, functions 60 of the radio access node 12 describedherein are implemented at the one or more processing nodes 50 ordistributed across the control system 34 (if present) and the one ormore processing nodes 50 in any desired manner. In some particularembodiments, some or all of the functions 60 of the radio access node 12described herein are implemented as virtual components executed by oneor more virtual machines implemented in a virtual environment(s) hostedby the processing node(s) 50. As will be appreciated by one of ordinaryskill in the art, additional signaling or communication between theprocessing node(s) 50 and the control system 34 (if present) oralternatively the radio unit(s) 42 is used in order to carry out atleast some of the desired functions. Notably, in some embodiments, thecontrol system 34 may not be included, in which case the radio unit(s)42 communicates directly with the processing node(s) 50 via anappropriate network interface(s).

In some embodiments, a computer program including instructions which,when executed by at least one processor, causes the at least oneprocessor to carry out the functionality of the radio access node 12 ora processing node 50 according to any of the embodiments describedherein is provided. In some embodiments, a carrier containing theaforementioned computer program product is provided. The carrier is oneof an electronic signal, an optical signal, a radio signal, or acomputer readable storage medium (e.g., a non-transitory computerreadable medium such as memory).

FIG. 18 is a schematic block diagram of the radio access node 12according to some other embodiments of the present disclosure. The radioaccess node 12 includes one or more modules 62, each of which isimplemented in software. The module(s) 62 provide the functionality ofthe radio access node 12 described herein.

Example Embodiments

While not being limited thereto, some example embodiments of the presentdisclosure are provided below.

1. A method of operation of a second node (14) connected to a first node(12, 50) in a wireless communication network, comprising:

-   -   reporting (100A) rich CSI feedback to the first node (12, 50) on        a physical channel with a small payload.        2. The method of embodiment 1 wherein reporting the rich CSI        feedback comprises:    -   identifying (200A) a subset of codebook entries from a codebook        of coefficients;    -   selecting (202A) a codebook entry from the subset; and    -   reporting (204A) an index of the selected codebook entry.        3. The method of embodiment 2 wherein:    -   each entry of the codebook is identified by an index k    -   the entry of the codebook with index k comprises a vector or        matrix C_(k) of complex numbers with L′ rows and r columns, L′        and r being positive integers;    -   each of (L′−1)r elements of each entry comprise a scalar complex        number that can be one of N complex numbers;

C_(k₁) − C_(k₂)_(F) > 0where k₁≠k₂ are indices of different codebook entries, and ∥C∥_(F) isthe Frobenius norm of a matrix or vector C;

-   -   the codebook comprises N^((L′-1)r) entries; and    -   the subset comprises one of K^(M) entries, where K≤N and        M≤(L′-1)r are positive integers and each entry in the subset is        identified by an index.        4. The method of embodiment 3 wherein the selected codebook        entry for when r=2 can be constructed from M=2 distinct        variables and each variable can be one of K=N complex numbers        and C_(k) ^(H)C_(k)=I for each entry C_(k) in the subset.        5. The method of embodiment 2 wherein:    -   each entry of the codebook comprises a vector or matrix;    -   one or more elements of each entry comprise a scalar complex        number;    -   a norm between the matrix or vector difference between any two        different codebook entries is greater than zero.        6. The method of any one of embodiments 1 to 4 wherein the        selected codebook entry for when r=2 can be constructed from M=4        distinct variables and each variable can be one of K=√{square        root over (N)} complex numbers and C_(k) ^(H)C_(k)≠I for at        least one entry C_(k) in the subset.        7. A method of operation of a second node (14) connected to a        first node (12, 50) in a wireless communication network for        reporting multi-beam CSI, comprising:    -   reporting (300A) a rank indicator and a beam count indicator in        a first transmission on an uplink control channel; and    -   reporting (302A) a cophasing indicator in a second transmission        on the uplink control channel, the cophasing indicator        identifying a selected entry of a codebook of cophasing        coefficients wherein the number of bits in the cophasing        indicator is identified by at least one of the beam count        indicator and the rank indicator.        8. The method of embodiment 7 wherein the beam count indicator        comprises at least one of a number of beams and an indication of        relative powers, the possible values of the indication        comprising both a zero and a non-zero value.        9. A method of operation of a second node (14) connected to a        first node in a wireless communication network for reporting        CSI, comprising:    -   jointly identifying the number of beams and an index of a beam        in a multi-beam CSI report; and    -   transmitting the multi-beam CSI report to the first node (12,        50).        10. The method of embodiment 9 wherein jointly identifying the        number of beams and the index of the beam in the multi-beam CSI        report comprises:    -   determining (304A) a number of beams L used to construct the        multi-beam CSI report; and    -   determining (306A) a beam indicator for an l^(th) beam, the beam        indicator identifying the index of a beam of the multi-beam CSI        report if L is at least l, and otherwise identifying that L is        less than l.        11. A method of operation of a second node (14) connected to a        first node (12, 50) in a wireless communication network,        comprising:    -   reporting (400A) CSI corresponding to a first number of beams if        the CSI corresponds to a first rank; and    -   reporting (402A) CSI corresponding to a second number of beams        if the CSI corresponds to a second rank        12. The method of embodiment 11 wherein:    -   the first rank is smaller than the second rank; and    -   the first number of beams is larger than the second number of        beams.        13. The method of any one of embodiments 1 to 12 further        comprising:    -   providing an indication of at least one beam index pair index        (l_(k),m_(k)) in uplink control information, UCI, each beam        index pair corresponding to a beam k.        14. The method of any one of embodiments 1 to 13 wherein:    -   each beam is a k^(th) beam d(k) that comprises a set of complex        numbers and has index pair (l_(k),m_(k)), each element of the        set of complex numbers being characterized by at least one        complex phase shift such that:

d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))),

-   -   -   d_(n)(k), and d_(i)(k) are the l^(th) and n^(th) elements of            the beam d(k), respectively,        -   α_(i,n) is a real number corresponding to the i^(th) and            n^(th) elements of the beam d(k)        -   p and q are integers, and        -   beam directions Δ_(1,k) and Δ_(2,k) are real numbers            corresponding to beams with index pair (l_(k),m_(k)) that            determine the complex phase shifts e^(j2πΔ) ^(1,k) and            e^(j2πΔ) ^(2,k) respectively            15. The method of any one of embodiments 1 to 14 wherein the            first node (12, 50) is a radio access node (12).            16. The method of any one of embodiments 1 to 15 wherein the            second node (14) is a wireless device (14).            17. A second node (14) adapted to operate according to the            method of any one of embodiments 1 to 16.            18. A second node (14), comprising:

    -   at least one processor (20);

    -   memory (22) comprising instructions executable by the at least        one processor (20) whereby the second node (14) is operable to:        -   report rich CSI feedback to the first node (12, 50) on a            physical channel with a small payload.            19. A second node (14), comprising:

    -   a reporting module (32) operable to report rich CSI feedback to        the first node (12, 50) on a physical channel with a small        payload.        20. A method of operation of a first node (12) in a wireless        communication network, comprising:

    -   receiving (100B) rich CSI feedback from a second node (14) on a        physical channel with a small payload.        21. The method of embodiment 20 wherein reporting the rich CSI        feedback comprises:

    -   a subset of codebook being selected (200B) entries from a        codebook of coefficients;

    -   a codebook entry being selected (202B) from the subset; and

    -   receiving (204B) an index of the selected codebook entry.        22. The method of embodiment 21 wherein:

    -   each entry of the codebook is identified by an index k

    -   the entry of the codebook with index k comprises a vector or        matrix C_(k) of complex numbers with L′ rows and r columns, L′        and r being positive integers;

    -   each of (L′−1)r elements of each entry comprise a scalar complex        number that can be one of N complex numbers;

C_(k₁) − C_(k₂)_(F) > 0where k₁≠k₂ are indices of different codebook entries, and ∥C∥_(F) isthe Frobenius norm of a matrix or vector C;

-   -   the codebook comprises N^((L′-1)r) entries; and    -   the subset comprises one of K^(M) entries, where K≤N and        M<(L′−1)r are positive integers and each entry in the subset is        identified by an index.        23. The method of embodiment 22 wherein the selected codebook        entry for when r=2 can be constructed from M=2 distinct        variables and each variable can be one of K=N complex numbers        and C_(k) ^(H)C_(k)=I for each entry C_(k) in the subset.        24. The method of embodiment 21 wherein:    -   each entry of the codebook comprises a vector or matrix;    -   one or more elements of each entry comprise a scalar complex        number;    -   a norm between the matrix or vector difference between any two        different codebook entries is greater than zero.        25. The method of any one of embodiments 20 to 23 wherein the        selected codebook entry for when r=2 can be constructed from M=4        distinct variables and each variable can be one of K=√{square        root over (N)} complex numbers and C_(k) ^(H)C_(k)≠I for at        least one entry C_(k) in the subset.        26. A method of operation of a first node (12) in a wireless        communication network for reporting multi-beam CSI, comprising:    -   receiving (300B) a rank indicator and a beam count indicator in        a first transmission on an uplink control channel; and    -   receiving (302B) a cophasing indicator in a second transmission        on the uplink control channel, the cophasing indicator        identifying a selected entry of a codebook of cophasing        coefficients wherein the number of bits in the cophasing        indicator is identified by at least one of the beam count        indicator and the rank indicator.        27. The method of embodiment 26 wherein the beam count indicator        comprises at least one of a number of beams and an indication of        relative powers, the possible values of the indication        comprising both a zero and a non-zero value.        28. A method of operation of a first node (12) connected to a        first node in a wireless communication network for reporting        CSI, comprising:    -   jointly identifying the number of beams and an index of a beam        in a multi-beam CSI report; and    -   receiving the multi-beam CSI report from the second node (14).        29. The method of embodiment 28 wherein jointly identifying the        number of beams and the index of the beam in the multi-beam CSI        report comprises:    -   determining (304A) a number of beams L used to construct the        multi-beam CSI report; and    -   determining (306A) a beam indicator for an i^(th) beam, the beam        indicator identifying the index of a beam of the multi-beam CSI        report if L is at least l, and otherwise identifying that L is        less than l.        30. A method of operation of a first node (12) in a wireless        communication network, comprising:    -   receiving (400B) CSI corresponding to a first number of beams if        the CSI corresponds to a first rank; and    -   receiving (402B) CSI corresponding to a second number of beams        if the CSI corresponds to a second rank        31. The method of embodiment 30 wherein:    -   the first rank is smaller than the second rank; and    -   the first number of beams is larger than the second number of        beams.        32. The method of any one of embodiments 20 to 31 further        comprising:    -   receiving an indication of at least one beam index pair index        (l_(k),m_(k)) in uplink control information, UCI, each beam        index pair corresponding to a beam k.        33. The method of any one of embodiments 20 to 32 wherein:    -   each beam is a k^(th) beam d(k) that comprises a set of complex        numbers and has index pair (l_(k),m_(k)), each element of the        set of complex numbers being characterized by at least one        complex phase shift such that:

d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))),

-   -   -   d_(n)(k), and d_(i)(k) are the and n^(th) elements of the            beam d(k), respectively,        -   α_(i,n) is a real number corresponding to the i^(th) and            n^(th) elements of the beam d(k)        -   p and q are integers, and        -   beam directions Δ_(1,k) and Δ_(2,k) are real numbers            corresponding to beams with index pair (l_(k),m_(k)) that            determine the complex phase shifts e^(j2πΔ) ^(1,k) and            e^(j2πΔ) ^(2,k) respectively            34. The method of any one of embodiments 20 to 33 wherein            the first node (12, 50) is a radio access node (12).            35. The method of any one of embodiments 20 to 34 wherein            the second node (14) is a wireless device (14).            36. A first node (12) adapted to operate according to the            method of any one of embodiments 20 to 35.            37. A first node (12, 50), comprising:

    -   at least one processor (36);

    -   memory (38) comprising instructions executable by the at least        one processor (36) whereby the first node (12, 50) is operable        to:        -   receive rich CSI feedback from the second node (14) on a            physical channel with a small payload.            38. A first node (12, 50), comprising:

    -   a receiving module (62) operable to receive rich CSI feedback to        the first node (12, 50) on a physical channel with a small        payload.

The following acronyms are used throughout this disclosure.

-   -   1D One-Dimension    -   2D Two-Dimension    -   3GPP Third Generation Partnership Project    -   5G Fifth Generation    -   ACK Acknowledgement    -   ARQ Automatic Repeat-Request    -   ASIC Application Specific Integrated Circuit    -   BPSK Binary Phase-Shift Keying    -   CE Control Element    -   CPU Central Processing Unit    -   CQI Channel Quality Indicator    -   CRI CSI-RS Resource Indication    -   CSI Channel State Information    -   DCI Downlink Control Information    -   DFT Discrete Fourier Transform    -   DL-SCH Downlink Shared Channel    -   eNodeB Enhanced or Evolved Node B    -   EPDCCH Enhanced PDCCH    -   FDD Frequency Division Duplex    -   FD-MIMO Full Dimension MIMO    -   FPGA Field Programmable Gate Array    -   GSM Global System for Mobile Communications    -   HARQ Hybrid Automatic Repeat Request    -   LTE Long Term Evolution    -   MAC Medium Access Control    -   MCS Modulation And Coding State    -   MIMO Multiple-Input Multiple-Output    -   ms millisecond    -   MU-MIMO Multi-User MIMO    -   NACK Negative Acknowledgement    -   N_(R) New Radio    -   NZP Non-Zero Power    -   OFDM Orthogonal Frequency-Division Multiplexing    -   PDCCH Physical Downlink Control Channel    -   PMI Precoder Matrix Indicator    -   PRB Physical Resource Block    -   PUCCH Physical Uplink Control Channel    -   PUSCH Physical Uplink Shared Channel    -   QPSK Quadrature Phase-Shift Keying    -   RI Rank Indicator    -   RRC Radio Resource Control    -   RSRP Reference Signal Received Power    -   RSRQ Reference Signal Received Quality    -   RSSI Received Signal Strength Indicator    -   SINR Signal-to-Interference-and-Noise Ratio    -   SR Scheduling Request    -   SRB Signaling Radio Bearers    -   TDD Time-Division Duplex    -   TFRE Time/Frequency Resource Element    -   TS Technical Specification    -   UCI Uplink Control Information    -   UE User Equipment    -   ULA Uniform Linear Array    -   UL-SCH Uplink Shared Channel    -   UMB Ultra Mobile Broadband    -   UPA Uniform Planar Array    -   WCDMA Wideband Code-Division Multiple Access    -   WiMax Worldwide Interoperability for Microwave Access

Those skilled in the art will recognize improvements and modificationsto the embodiments of the present disclosure. All such improvements andmodifications are considered within the scope of the concepts disclosedherein.

What is claimed is:
 1. A method of operation of a second node connected to a first node in a wireless communication network for reporting multi-beam Channel State Information, CSI, comprising: reporting a rank indicator and a beam count indicator in a first transmission to the first node; and reporting a cophasing indicator in a second transmission to the first node, the cophasing indicator identifying a selected entry of a codebook of cophasing coefficients wherein a number of bits in the cophasing indicator is identified by at least one of the beam count indicator and the rank indicator.
 2. The method of claim 1 wherein: reporting the rank indicator and the beam count indicator in the first transmission comprises reporting the rank indicator and the beam count indicator in the first transmission on an uplink control channel; and reporting the cophasing indicator in the second transmission comprises reporting the cophasing indicator in the second transmission on the uplink control channel.
 3. The method of claim 1 wherein the beam count indicator comprises at least one of the group consisting of: a number of beams and an indication of relative powers.
 4. The method of claim 1 wherein possible values of at least one of the beam count indicator, and the cophasing indicator comprise both a zero and a non-zero value.
 5. The method of claim 1 further comprising: reporting a beam index in a third transmission to the first node.
 6. The method of claim 5 wherein the third transmission also comprises at least one of the group consisting of a beam rotation and second beam index.
 7. The method of claim 5 further comprising: jointly identifying the number of beams and indices of the beams in a multi-beam CSI report; and the first transmission, the second transmission, and the third transmission comprise transmitting the multi-beam CSI report to the first node.
 8. The method of claim 7 wherein jointly identifying the number of beams and the indices of the beams in the multi-beam CSI report comprises: determining a number of beams L used to construct the multi-beam CSI report; and determining a beam indicator for an l^(th) beam, the beam indicator identifying the index of the l^(th) beam of the multi-beam CSI report if L is at least l, and otherwise identifying that L is less than l.
 9. The method of claim 5, wherein reporting a beam index further comprising: providing an indication of at least one beam index pair index (l_(k),m_(k)) corresponding to a beam d(k).
 10. The method of claim 1 further comprising: reporting CSI corresponding to a first number of beams if the CSI corresponds to a first rank; and reporting CSI corresponding to a second number of beams if the CSI corresponds to a second rank.
 11. The method of claim 10 wherein: the first rank is smaller than the second rank; and the first number of beams is larger than the second number of beams.
 12. The method of claim 1 wherein: each beam d(k) comprises a set of complex numbers and, each element of the set of complex numbers is characterized by at least one complex phase shift such that: d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))), d_(n)(k), and d_(i)(k) are the n^(th) and i^(th) elements of the beam d(k), respectively, α_(i,n) is a real number corresponding to the i^(th) and n^(th) elements of the beam d(k) p and q are integers, and Δ_(1,k) and Δ_(2,k) are real numbers corresponding to a two-dimensional beam with index pair (l_(k),m_(k)) that determine the complex phase shifts e^(j2πΔ) ^(1,k) and e^(j2πΔ) ^(2,k) in each dimension, respectively.
 13. The method of claim 1 wherein the first node is a radio access node.
 14. The method of claim 1 wherein the second node is a wireless device.
 15. The method of claim 1 wherein the wireless communication network is a Long Term Evolution, LTE, wireless communication network.
 16. A method of operation of a first node connected to a second node in a wireless communication network for receiving multi-beam Channel State Information, CSI, comprising: receiving a rank indicator and a beam count indicator in a first transmission from the second node; and receiving a cophasing indicator in a second transmission from the second node, the cophasing indicator identifying a selected entry of a codebook of cophasing coefficients wherein a number of bits in the cophasing indicator is identified by at least one of the beam count indicator and the rank indicator.
 17. The method of claim 16 wherein: receiving the rank indicator and the beam count indicator in the first transmission comprises receiving the rank indicator and the beam count indicator in the first transmission on an uplink control channel; and receiving the cophasing indicator in the second transmission comprises receiving the cophasing indicator in the second transmission on the uplink control channel.
 18. The method of claim 16 wherein the beam count indicator comprises at least one of the group consisting of: a number of beams and an indication of relative powers.
 19. The method of claim 16 wherein possible values of at least one of the beam count indicator, and the cophasing indicator comprise both a zero and a non-zero value.
 20. The method of claim 16 further comprising: receiving a beam index in a third transmission from the second node.
 21. The method of claim 20 wherein the third transmission also comprises at least one of the group consisting of a beam rotation and second beam index.
 22. The method of claim 16 further comprising: receiving CSI corresponding to a first number of beams if the CSI corresponds to a first rank; and receiving CSI corresponding to a second number of beams if the CSI corresponds to a second rank.
 23. The method of claim 22 wherein: the first rank is smaller than the second rank; and the first number of beams is larger than the second number of beams.
 24. The method of claim 20 further comprising: receiving an indication of at least one beam index pair index (l_(k),m_(k)) corresponding to a beam k.
 25. The method of claim 16 wherein: each beam d(k) comprises a set of complex numbers and, each element of the set of complex numbers is characterized by at least one complex phase shift such that: d_(n)(k) = d_(i)(k)α_(i, n)e^(j 2π(p Δ_(1, k) + q Δ_(2, k))), d_(n)(k), and d_(i)(k) are the n^(th) and i^(th) elements of the beam d(k), respectively, α_(i,n) is a real number corresponding to the i^(th) and n^(th) elements of the beam d(k) p and q are integers, and Δ_(1,k) and Δ_(2,k) are real numbers corresponding to a two-dimensional beam with index pair (l_(k),m_(k)) that determine the complex phase shifts e^(j2πΔ) ^(1,k) and e^(j2πΔ) ^(2,k) in each dimension respectively.
 26. The method of claim 16 wherein the first node is a radio access node.
 27. The method of claim 16 wherein the second node is a wireless device.
 28. The method of claim 16 wherein the wireless communication network is a Long Term Evolution, LTE, wireless communication network.
 29. A second node, comprising: at least one processor; memory comprising instructions executable by the at least one processor whereby the second node is operable to: report a rank indicator and a beam count indicator in a first transmission to a first node; and report a cophasing indicator in a second transmission to the first node, the cophasing indicator identifying a selected entry of a codebook of cophasing coefficients wherein a number of bits in the cophasing indicator is identified by at least one of the beam count indicator and the rank indicator.
 30. A first node, comprising: at least one processor; memory comprising instructions executable by the at least one processor whereby the first node is operable to: receive a rank indicator and a beam count indicator in a first transmission from a second node; and receive a cophasing indicator in a second transmission from the second node, the cophasing indicator identifying a selected entry of a codebook of cophasing coefficients wherein a number of bits in the cophasing indicator is identified by at least one of the beam count indicator and the rank indicator. 